Don't unconditionally define feature test macros in <version>

Don't unconditionally define feature test macros in <version>

The macro definitions in <version> should depend on the same
preprocessor conditions as the original macros in other headers.
Otherwise <version> can define macros that imply the availability of
features that are not actually defined.

This fix is incomplete, as __cpp_lib_filesystem should depend on whether
libstdc++fs.a is supported, and several macros should only be defined
when _GLIBCXX_HOSTED is defined. Also, the feature test macros should
define their value as type long, but most are type int.

	* include/bits/c++config (_GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP)
	(_GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE): Move definitions here.
	(_GLIBCXX_HAVE_BUILTIN_LAUNDER): Likewise. Use !__is_identifier
	instead of __has_builtin.
	* include/std/type_traits (_GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP)
	(_GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE): Remove definitions from here.
	* include/std/version [!_GLIBCXX_HAS_GTHREADS]
	(__cpp_lib_shared_timed_mutex, __cpp_lib_scoped_lock)
	(__cpp_lib_shared_mutex): Don't define when Gthreads not in use.
	[!_GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP]
	(__cpp_lib_has_unique_object_representations): Don't define when
	builtin not available.
	[!_GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE] (__cpp_lib_is_aggregate):
	Likewise.
	[!_GLIBCXX_HAVE_BUILTIN_LAUNDER] (__cpp_lib_launder): Likewise.
	* libsupc++/new (_GLIBCXX_HAVE_BUILTIN_LAUNDER): Remove definition
	from here.

From-SVN: r263184