AC_PROG_CC_STDC: fold into AC_PROG_CC, removing C11 macro

AC_PROG_CC_STDC: fold into AC_PROG_CC, removing C11 macro

* NEWS:
* doc/autoconf.texi (C Compiler): Document the following.
* lib/autoconf/c.m4 (AC_PROG_CC): Check for the latest C version
supported, not just C89.
(_AC_C_STD_TRY): Keep track of the options we add to bring
the C compiler up to standard, so that we can undo it if the
user later requests some other C standard.
(_AC_PROG_CC_FORCE_VERSION): New macro.
(AC_PROG_CC_C89, AC_PROG_CC_C99): Use it.  These macros now
have a documented side effect of changing the C version requested.
(AC_PROG_CC_C11): Remove.  It wasn't useful.
(AC_PROG_CC_STDC): Now an obsolescent alias for AC_PROG_CC.
(AC_C_PROTOTYPES): Allow any standard C version, not just c89.
Don't chatter, since we don't actually run any checking code.
* lib/autoconf/types.m4 (AC_TYPE_LONG_LONG_INT)
(AC_TYPE_UNSIGNED_LONG_LONG_INT): Treat C11 like C99.
* TODO: Remove the TODO item corresponding to the above.