libstdc++: Replace AC_CACHE_VAL with AC_CACHE_CHECK

This replaces most uses of AC_CACHE_VAL with AC_CACHE_CHECK, which means
we don't need separate AC_MSG_CHECKING and AC_MSG_RESULT macros.

There are a few trivial bugs fixed as a side effect, where an
AC_MSG_RESULT was printed out even if the actual checks hadn't been
done. That didn't affect the results, only the content of config.log.

libstdc++-v3/ChangeLog:

	* acinclude.m4: Replace AC_CACHE_VAL with AC_CACHE_CHECK.
	* configure: Regenerate.