OpenWatcom 1.8 has a C99 mode, which implements _Bool and
<stdbool.h>. Unfortunately the implementation is broken, and doesn't
let _Bool be implicitly converted to integer (as required by the C99
spec). Detect this case in autoconf.
We can actually test for the options being accepted, rather than try
to test for gcc; this handles differences between gcc versions as well
as compilers with a similar command line set.
