src: Fix checking for clang

src: Fix checking for clang

The check was trying to use the shell variable $CC instead of
the make variable $(CC); it also interpreted grep's return code
wrong: 1 means the provided pattern was *not* matched. As a
result, pdwtags was never run, not even when building with gcc.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>