Make sure we test valid C99 programs.

In particular, Clang is very fussy about not having this. It doesn't
really hurt anything, but it's better to suppress the annoying warning
properly.

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Merge pull request #1376 from thoughtpolice/clang-tweak

Make sure we test valid C99 programs.