ssp: rework the logic to use priority=200 on clang builds

The preproc logic was added at the last minute to appease GCC 4.2, and
kevans@ did clearly not go back and double-check that the logic worked out
for clang builds to use the new variant.

It turns out that clang defines GNUC == 4. Flip it around and check
clang as well, leaving a note to remove it later.

Reported by: cem