libstdc++: Improve config output for --enable-cstdio [PR104301]

Currently we just print "checking for underlying I/O to use... stdio"
unconditionally, whether configured to use stdio_pure or stdio_posix. We
should make it clear that the user's configure option chose the right
thing.

libstdc++-v3/ChangeLog:

	PR libstdc++/104301
	* acinclude.m4 (GLIBCXX_ENABLE_CSTDIO): Print different messages
	for stdio_pure and stdio_posix options.
	* configure: Regenerate.