meson: Don't build tests when CLang lacks -fsemantic-interposition

There are some CLang versions that do not support
-fsemantic-interposition. If that's the case, the code is
optimized so much that our mocking no longer works.

Therefore, disable tests and produce a warning.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>