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.