libstdc++: Override detection of flockfile support in newlib [PR123406]

libstdc++: Override detection of flockfile support in newlib [PR123406]

As explained in the PR, flockfile and funlockfile are always declared by
newlib and there's no easy way to detect whether they're actually
defined. Ensure that ac_stdio_locking=no gets set for non-cygwin newlib
targets.

libstdc++-v3/ChangeLog:

	PR libstdc++/123406
	* acinclude.m4 (GLIBCXX_CHECK_STDIO_LOCKING): Override detection
	of flockfile for non-cygwin newlib targets.
	* configure: Regenerate.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>