AC_PROG_CC_C11: new macro, which AC_PROG_CC_STDC now defaults to

AC_PROG_CC_C11: new macro, which AC_PROG_CC_STDC now defaults to

* NEWS:
* doc/autoconf.texi (C Compiler): Document this.
(Gnulib, Function Portability, Particular Functions)
(Header Portability, Particular Headers, Defining Symbols)
(Printing Messages, Limitations of Usual Tools)
(Preprocessor Arithmetic, Volatile Objects, Exiting Portably):
Modernize wording for C11.
* lib/autoconf/c.m4 (_AC_C_C99_TEST_HEADER, _AC_C_C99_TEST_BODY):
New macros, taken from _AC_PROG_CC_C99.  These are so that we can
also include the C99 tests in the C11 test program.
(_AC_PROG_CC_C99): Use them.
(_AC_PROG_CC_C11, AC_PROG_CC_C11): New macros.
(AC_PROG_CC_STDC): Prefer C11 to C99 or C89.