libstdc++: Test errno macros directly, not via autoconf [PR 93151]

This fixes a bug caused by a mismatch between the macros defined by
<errno.h> when GCC is built and the macros defined by <errno.h> when
users include <system_error>. If the user code is compiled with
_XOPEN_SOURCE defined to 500 or 600, Darwin suppresses the
ENOTRECOVERABLE and EOWNERDEAD macros, which are not defined by SUSv3
(aka POSIX.1-2001).

Since POSIX requires the errno macros to be macros (and not variables or
enumerators) we can just test for them directly using the preprocessor.
That means that <system_error> will match what is actuallydefined when
it's included, not what was defined when GCC was built. With that change
there is no need for the GLIBCXX_CHECK_SYSTEM_ERROR configure checks and
they can be removed.

libstdc++-v3/ChangeLog:

	PR libstdc++/93151
	* acinclude.m4 (GLIBCXX_CHECK_SYSTEM_ERROR): Remove.
	* configure.ac: Regenerate.
	* config/os/generic/error_constants.h: Test POSIX errno macros
	directly, instead of corresponding _GLIBCXX_HAVE_EXXX macros.
	* testsuite/19_diagnostics/headers/system_error/errc_std_c++0x.cc:
	Likewise.
	* testsuite/19_diagnostics/headers/system_error/93151.cc: New
	test.