libstdc++: Make C++17 ignore --disable-libstdcxx-filesystem-ts [PR 94681]

The configure switch should only affect the optional Filesystem TS, not
the std::filesystem features of C++17.

libstdc++-v3/ChangeLog:

	PR libstdc++/94681
	* acinclude.m4 (GLIBCXX_CHECK_FILESYSTEM_DEPS): Do not depend on
	$enable_libstdcxx_filesystem_ts.
	* configure: Regenerate.