Use -Wabi=2 to fix warnings about -Wabi having no effect on its own.
This requires suppressing two warnings in src/c++11/debug.cc which do
not affect the library ABI.

Previously libstdc++ defaulted to --enable-werror but the -Werror flag
was not actually added unless --enable-maintainer-mode was used. This is
not documented and not the expected behaviour. This removes any special
treatment for maintainer-mode, makes -Werror depend directly on
--enable-werror, and changes the default to --enable-werror=no.

	PR libstdc++/86450
	* acinclude.m4 (GLIBCXX_CHECK_COMPILER_FEATURES): Don't define WERROR.
	(GLIBCXX_EXPORT_FLAGS): Use -Wabi=2 instead of -Wabi.
	* configure: Regenerate.
	* configure.ac: Change GLIBCXX_ENABLE_WERROR default to "no".
	* doc/Makefile.in: Regenerate.
	* fragment.am: Set WERROR_FLAG to -Werror instead of $(WERROR).
	* include/Makefile.in: Regenerate.
	* libsupc++/Makefile.in: Regenerate.
	* po/Makefile.in: Regenerate.
	* python/Makefile.in: Regenerate.
	* src/Makefile.in: Regenerate.
	* src/c++11/Makefile.in: Regenerate.
	* src/c++11/debug.cc: Use diagnostic pragmas to suppress warnings
	from -Wabi=2 that don't affect exported symbols.
	* src/c++98/Makefile.in: Regenerate.
	* src/filesystem/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.

From-SVN: r262824