Avoid -O2 in AC_PROG_CC/CXX, work around occasional Clang 12.0.1 SEGVs

Avoid -O2 in AC_PROG_CC/CXX, work around occasional Clang 12.0.1 SEGVs

At least with one ASan/UBSan setup of mine using LODE's Clang 12.0.1,
./autogen.sh would occasionally detect -std=gnu11 as the required CC "option to
enable C11 features" (which would in turn cause building external/firebird to
fail oddly; an issue worth investigations of its own), because Clang would
occasionally crash with a SEGV on the corresponding configure test program's
first invocation (without -std=gnu11) when invoked with -O2 (and happen to
succeed on second invocation with -std=gnu11, so configure thinks that's
needed), see below for a relevant config.log excerpt.

When CC/CXX are already set (as is the case in this scenario), we could arguably
skip the AC_PROG_CC/CXX checks entirely (and thus avoid configure potentially
adding -std=gnu11 to CC), but at least AC_PROG_CC also internally sets the GCC
shell var, which we use in configure.ac.  So better be conservative and just
avoid -O2 during AC_PROG_CC/CXX (whatever the autoconf motivation to include it
in the first place).

> configure:8165: checking for /home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang -fsanitize=address -fsanitize=undefined -fsanitize=float-divide-by-zero -fsanitize=local-bounds -fsanitize-blacklist=/home/sberg/lo0/core/sanitize-ubsan-excludelist option to enable C11 features
> configure:8180: /home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang -fsanitize=address -fsanitize=undefined -fsanitize=float-divide-by-zero -fsanitize=local-bounds -fsanitize-blacklist=/home/sberg/lo0/core/sanitize-ubsan-excludelist  -c -g -O2  conftest.c >&5
> PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace, preprocessed source, and associated run script.
> Stack dump:
> 0.	Program arguments: /home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang -fsanitize=address -fsanitize=undefined -fsanitize=float-divide-by-zero -fsanitize=local-bounds -fsanitize-blacklist=/home/sberg/lo0/core/sanitize-ubsan-excludelist -c -g -O2 conftest.c
> 1.	<eof> parser at end of file
> 2.	Code generation
>  #0 0x000055f3a890caf2 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x1c3eaf2)
>  #1 0x000055f3a890a734 llvm::sys::RunSignalHandlers() (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x1c3c734)
>  #2 0x000055f3a887b998 CrashRecoverySignalHandler(int) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x1bad998)
>  #3 0x00007f750d24e520 (/lib/x86_64-linux-gnu/libc.so.6+0x42520)
>  #4 0x000055f3a93f9cd4 llvm::DIE::getUnitDie() const (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x272bcd4)
>  #5 0x000055f3a9404574 llvm::DwarfDebug::finishEntityDefinitions() (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x2736574)
>  #6 0x000055f3a941df99 llvm::DwarfDebug::finalizeModuleInfo() (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x274ff99)
>  #7 0x000055f3a9421128 llvm::DwarfDebug::endModule() (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x2753128)
>  #8 0x000055f3a93f1219 llvm::AsmPrinter::doFinalization(llvm::Module&) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x2723219)
>  #9 0x000055f3a82478f5 llvm::FPPassManager::doFinalization(llvm::Module&) (.localalias) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x15798f5)
> #10 0x000055f3a8253900 llvm::legacy::PassManagerImpl::run(llvm::Module&) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x1585900)
> #11 0x000055f3a8bb57d3 (anonymous namespace)::EmitAssemblyHelper::EmitAssembly(clang::BackendAction, std::unique_ptr<llvm::raw_pwrite_stream, std::default_delete<llvm::raw_pwrite_stream> >) (.constprop.0) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x1ee77d3)
> #12 0x000055f3a8bb76ea clang::EmitBackendOutput(clang::DiagnosticsEngine&, clang::HeaderSearchOptions const&, clang::CodeGenOptions const&, clang::TargetOptions const&, clang::LangOptions const&, llvm::DataLayout const&, llvm::Module*, clang::BackendAction, std::unique_ptr<llvm::raw_pwrite_stream, std::default_delete<llvm::raw_pwrite_stream> >) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x1ee96ea)
> #13 0x000055f3a9825876 clang::BackendConsumer::HandleTranslationUnit(clang::ASTContext&) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x2b57876)
> #14 0x000055f3aa35c549 clang::ParseAST(clang::Sema&, bool, bool) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x368e549)
> #15 0x000055f3a91ef2d9 clang::FrontendAction::Execute() (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x25212d9)
> #16 0x000055f3a91903a3 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x24c23a3)
> #17 0x000055f3a92a2fd8 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x25d4fd8)
> #18 0x000055f3a7798815 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0xaca815)
> #19 0x000055f3a77961a7 ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0xac81a7)
> #20 0x000055f3a9039689 void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<llvm::Optional<llvm::StringRef> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, bool*) const::'lambda'()>(long) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x236b689)
> #21 0x000055f3a887baa7 llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x1badaa7)
> #22 0x000055f3a903a802 clang::driver::CC1Command::Execute(llvm::ArrayRef<llvm::Optional<llvm::StringRef> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, bool*) const (.part.0) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x236c802)
> #23 0x000055f3a900fbec clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&) const (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x2341bec)
> #24 0x000055f3a9010679 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*> >&) const (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x2342679)
> #25 0x000055f3a901e8f1 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*> >&) (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0x23508f1)
> #26 0x000055f3a7717e6f main (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0xa49e6f)
> #27 0x00007f750d235d90 (/lib/x86_64-linux-gnu/libc.so.6+0x29d90)
> #28 0x00007f750d235e40 __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x29e40)
> #29 0x000055f3a7795b55 _start (/home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang+0xac7b55)
> clang-12: error: clang frontend command failed with exit code 139 (use -v to see invocation)
> clang version 12.0.1 (https://github.com/llvm/llvm-project.git fed41342a82f5a3a9201819a82bf7a48313e296b)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
> InstalledDir: /home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin
> clang-12: note: diagnostic msg:
> ********************
>
> PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
> Preprocessed source(s) and associated run script(s) are located at:
> clang-12: note: diagnostic msg: /tmp/conftest-968380.c
> clang-12: note: diagnostic msg: /tmp/conftest-968380.sh
> clang-12: note: diagnostic msg:
>
> ********************
> configure:8180: $? = 139
> configure: failed program was:
> | /* confdefs.h */
> | #define PACKAGE_NAME "LibreOffice"
> | #define PACKAGE_TARNAME "libreoffice"
> | #define PACKAGE_VERSION "24.8.0.0.alpha0+"
> | #define PACKAGE_STRING "LibreOffice 24.8.0.0.alpha0+"
> | #define PACKAGE_BUGREPORT ""
> | #define PACKAGE_URL "http://documentfoundation.org/"
> | #define LIBO_VERSION_MAJOR 24
> | #define LIBO_VERSION_MINOR 8
> | #define LIBO_VERSION_MICRO 0
> | #define LIBO_VERSION_PATCH 0
> | #define LIBO_THIS_YEAR 2024
> | #define SRCDIR "/home/sberg/lo0/core"
> | #define SRC_ROOT "/home/sberg/lo0/core"
> | #define BUILDDIR "/home/sberg/lo0/core"
> | #define USE_HEADLESS_CODE 1
> | #define ENABLE_HEADLESS 1
> | /* end confdefs.h.  */
> |
> | /* Does the compiler advertise C89 conformance?
> |    Do not test the value of __STDC__, because some compilers set it to 0
> |    while being otherwise adequately conformant. */
> | #if !defined __STDC__
> | # error "Compiler does not advertise C89 conformance"
> | #endif
> |
> | #include <stddef.h>
> | #include <stdarg.h>
> | struct stat;
> | /* Most of the following tests are stolen from RCS 5.7 src/conf.sh.  */
> | struct buf { int x; };
> | struct buf * (*rcsopen) (struct buf *, struct stat *, int);
> | static char *e (p, i)
> |      char **p;
> |      int i;
> | {
> |   return p[i];
> | }
> | static char *f (char * (*g) (char **, int), char **p, ...)
> | {
> |   char *s;
> |   va_list v;
> |   va_start (v,p);
> |   s = g (p, va_arg (v,int));
> |   va_end (v);
> |   return s;
> | }
> |
> | /* OSF 4.0 Compaq cc is some sort of almost-ANSI by default.  It has
> |    function prototypes and stuff, but not \xHH hex character constants.
> |    These do not provoke an error unfortunately, instead are silently treated
> |    as an "x".  The following induces an error, until -std is added to get
> |    proper ANSI mode.  Curiously \x00 != x always comes out true, for an
> |    array size at least.  It is necessary to write \x00 == 0 to get something
> |    that is true only with -std.  */
> | int osf4_cc_array ['\x00' == 0 ? 1 : -1];
> |
> | /* IBM C 6 for AIX is almost-ANSI by default, but it replaces macro parameters
> |    inside strings and character constants.  */
> | #define FOO(x) 'x'
> | int xlc6_cc_array[FOO(a) == 'x' ? 1 : -1];
> |
> | int test (int i, double x);
> | struct s1 {int (*f) (int a);};
> | struct s2 {int (*f) (double a);};
> | int pairnames (int, char **, int *(*)(struct buf *, struct stat *, int),
> |                int, int);
> |
> | // Does the compiler advertise C99 conformance?
> | #if !defined __STDC_VERSION__ || __STDC_VERSION__ < 199901L
> | # error "Compiler does not advertise C99 conformance"
> | #endif
> |
> | #include <stdbool.h>
> | extern int puts (const char *);
> | extern int printf (const char *, ...);
> | extern int dprintf (int, const char *, ...);
> | extern void *malloc (size_t);
> |
> | // Check varargs macros.  These examples are taken from C99 6.10.3.5.
> | // dprintf is used instead of fprintf to avoid needing to declare
> | // FILE and stderr.
> | #define debug(...) dprintf (2, __VA_ARGS__)
> | #define showlist(...) puts (#__VA_ARGS__)
> | #define report(test,...) ((test) ? puts (#test) : printf (__VA_ARGS__))
> | static void
> | test_varargs_macros (void)
> | {
> |   int x = 1234;
> |   int y = 5678;
> |   debug ("Flag");
> |   debug ("X = %d\n", x);
> |   showlist (The first, second, and third items.);
> |   report (x>y, "x is %d but y is %d", x, y);
> | }
> |
> | // Check long long types.
> | #define BIG64 18446744073709551615ull
> | #define BIG32 4294967295ul
> | #define BIG_OK (BIG64 / BIG32 == 4294967297ull && BIG64 % BIG32 == 0)
> | #if !BIG_OK
> |   #error "your preprocessor is broken"
> | #endif
> | #if BIG_OK
> | #else
> |   #error "your preprocessor is broken"
> | #endif
> | static long long int bignum = -9223372036854775807LL;
> | static unsigned long long int ubignum = BIG64;
> |
> | struct incomplete_array
> | {
> |   int datasize;
> |   double data[];
> | };
> |
> | struct named_init {
> |   int number;
> |   const wchar_t *name;
> |   double average;
> | };
> |
> | typedef const char *ccp;
> |
> | static inline int
> | test_restrict (ccp restrict text)
> | {
> |   // See if C++-style comments work.
> |   // Iterate through items via the restricted pointer.
> |   // Also check for declarations in for loops.
> |   for (unsigned int i = 0; *(text+i) != '\0'; ++i)
> |     continue;
> |   return 0;
> | }
> |
> | // Check varargs and va_copy.
> | static bool
> | test_varargs (const char *format, ...)
> | {
> |   va_list args;
> |   va_start (args, format);
> |   va_list args_copy;
> |   va_copy (args_copy, args);
> |
> |   const char *str = "";
> |   int number = 0;
> |   float fnumber = 0;
> |
> |   while (*format)
> |     {
> |       switch (*format++)
> | 	{
> | 	case 's': // string
> | 	  str = va_arg (args_copy, const char *);
> | 	  break;
> | 	case 'd': // int
> | 	  number = va_arg (args_copy, int);
> | 	  break;
> | 	case 'f': // float
> | 	  fnumber = va_arg (args_copy, double);
> | 	  break;
> | 	default:
> | 	  break;
> | 	}
> |     }
> |   va_end (args_copy);
> |   va_end (args);
> |
> |   return *str && number && fnumber;
> | }
> |
> |
> | // Does the compiler advertise C11 conformance?
> | #if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112L
> | # error "Compiler does not advertise C11 conformance"
> | #endif
> |
> | // Check _Alignas.
> | char _Alignas (double) aligned_as_double;
> | char _Alignas (0) no_special_alignment;
> | extern char aligned_as_int;
> | char _Alignas (0) _Alignas (int) aligned_as_int;
> |
> | // Check _Alignof.
> | enum
> | {
> |   int_alignment = _Alignof (int),
> |   int_array_alignment = _Alignof (int[100]),
> |   char_alignment = _Alignof (char)
> | };
> | _Static_assert (0 < -_Alignof (int), "_Alignof is signed");
> |
> | // Check _Noreturn.
> | int _Noreturn does_not_return (void) { for (;;) continue; }
> |
> | // Check _Static_assert.
> | struct test_static_assert
> | {
> |   int x;
> |   _Static_assert (sizeof (int) <= sizeof (long int),
> |                   "_Static_assert does not work in struct");
> |   long int y;
> | };
> |
> | // Check UTF-8 literals.
> | #define u8 syntax error!
> | char const utf8_literal[] = u8"happens to be ASCII" "another string";
> |
> | // Check duplicate typedefs.
> | typedef long *long_ptr;
> | typedef long int *long_ptr;
> | typedef long_ptr long_ptr;
> |
> | // Anonymous structures and unions -- taken from C11 6.7.2.1 Example 1.
> | struct anonymous
> | {
> |   union {
> |     struct { int i; int j; };
> |     struct { int k; long int l; } w;
> |   };
> |   int m;
> | } v1;
> |
> |
> | int
> | main (int argc, char **argv)
> | {
> |   int ok = 0;
> |
> | ok |= (argc == 0 || f (e, argv, 0) != argv[0] || f (e, argv, 1) != argv[1]);
> |
> |
> |   // Check bool.
> |   _Bool success = false;
> |   success |= (argc != 0);
> |
> |   // Check restrict.
> |   if (test_restrict ("String literal") == 0)
> |     success = true;
> |   char *restrict newvar = "Another string";
> |
> |   // Check varargs.
> |   success &= test_varargs ("s, d' f .", "string", 65, 34.234);
> |   test_varargs_macros ();
> |
> |   // Check flexible array members.
> |   struct incomplete_array *ia =
> |     malloc (sizeof (struct incomplete_array) + (sizeof (double) * 10));
> |   ia->datasize = 10;
> |   for (int i = 0; i < ia->datasize; ++i)
> |     ia->data[i] = i * 1.234;
> |
> |   // Check named initializers.
> |   struct named_init ni = {
> |     .number = 34,
> |     .name = L"Test wide string",
> |     .average = 543.34343,
> |   };
> |
> |   ni.number = 58;
> |
> |   int dynamic_array[ni.number];
> |   dynamic_array[0] = argv[0][0];
> |   dynamic_array[ni.number - 1] = 543;
> |
> |   // work around unused variable warnings
> |   ok |= (!success || bignum == 0LL || ubignum == 0uLL || newvar[0] == 'x'
> | 	 || dynamic_array[ni.number - 1] != 543);
> |
> |
> |   _Static_assert ((offsetof (struct anonymous, i)
> | 		   == offsetof (struct anonymous, w.k)),
> | 		  "Anonymous union alignment botch");
> |   v1.i = 2;
> |   v1.w.k = 5;
> |   ok |= v1.i != 5;
> |
> |   return ok;
> | }
> |
> configure:8180: /home/builder/lode/opt_private/clang-llvmorg-12.0.1/bin/clang -fsanitize=address -fsanitize=undefined -fsanitize=float-divide-by-zero -fsanitize=local-bounds -fsanitize-blacklist=/home/sberg/lo0/core/sanitize-ubsan-excludelist -std=gnu11 -c -g -O2  conftest.c >&5
> configure:8180: $? = 0
> configure:8201: result: -std=gnu11

Change-Id: I9122d0d853d0010155d57cb1d1d56f7c453d5208
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/165904
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>