libcpp: Add missing config for --enable-valgrind-annotations [PR107691]

r7-912 copied (parts of) the valgrind annotation checks from gcc
to libcpp. The above copies the missing pieces to libcpp to diagnose
when libcpp is configured with --enable-valgrind-annotations but
valgrind is not installed.

libcpp/ChangeLog:

	PR preprocessor/107691
	* configure.ac: Add valgrind header checks.
	* configure: Regenerate.