Fix AIX libstdc++ semaphore support [PR100164]

> > The #error would not be hit if _GLIBCXX_HAVE_POSIX_SEMAPHORE were defined,
> > but it shows up in your error report.

> You now have pinpointed the problem.

> It's not that AIX doesn't have semaphore, but that the code previously
> had a fallback that hid a bug in the macros:

  // Use futex if available and didn't force use of POSIX
  using __fast_semaphore = __atomic_semaphore<__detail::__platform_wait_t>;
  using __fast_semaphore = __platform_semaphore;
  using __fast_semaphore = __atomic_semaphore<ptrdiff_t>;

> The problem is that libstdc++ configure defines
> _GLIBCXX_HAVE_POSIX_SEMAPHORE in config.h.  libstdc++ uses sed to
> rewrite config.h to c++config.h and prepends _GLIBCXX_, so c++config.h
> contains

> And bits/semaphore_base.h is not testing that corrupted macro.  Either
> semaphore_base.h needs to test for the corrupted macro, or libtsdc++
> configure needs to define HAVE_POSIX_SEMAPHORE without itself
> prepending _GLIBCXX_  so that the c++config.h rewriting works
> correctly and defines the correct macro for semaphore_base.h.

The include/Makefile.am sed is:
        sed -e 's/HAVE_/_GLIBCXX_HAVE_/g' \
            -e 's/PACKAGE/_GLIBCXX_PACKAGE/g' \
            -e 's/VERSION/_GLIBCXX_VERSION/g' \
            -e 's/WORDS_/_GLIBCXX_WORDS_/g' \
            -e 's/_DARWIN_USE_64_BIT_INODE/_GLIBCXX_DARWIN_USE_64_BIT_INODE/g' \
            -e 's/_FILE_OFFSET_BITS/_GLIBCXX_FILE_OFFSET_BITS/g' \
            -e 's/_LARGE_FILES/_GLIBCXX_LARGE_FILES/g' \
            -e 's/ICONV_CONST/_GLIBCXX_ICONV_CONST/g' \
            -e '/[       ]_GLIBCXX_LONG_DOUBLE_COMPAT[   ]/d' \
            -e '/[       ]_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT[    ]/d' \
            < ${CONFIG_HEADER} >> $@ ;\
so for many macros one needs _GLIBCXX_ prefixes already in configure,
as can be seen in grep AC_DEFINE.*_GLIBCXX configure.ac acinclude.m4
But _GLIBCXX_HAVE_POSIX_SEMAPHORE is the only one that shouldn't have
that prefix because the sed is adding that.
E.g. on i686-linux, I see
grep _GLIBCXX__GLIBCXX c++config.h
that proves it is the only broken one.

So this change fixes the acinclude.m4 side.

2021-04-21  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/100164
	* acinclude.m4: For POSIX semaphores AC_DEFINE HAVE_POSIX_SEMAPHORE
	rather than _GLIBCXX_HAVE_POSIX_SEMAPHORE.
	* configure: Regenerated.
	* config.h.in: Regenerated.