Avoid spurious GCC warnings in STATIC_ASSERT() macro.

Avoid spurious GCC warnings in STATIC_ASSERT() macro.

GCC 4.8 now warns about typedefs that are local to a scope and not
used anywhere within that scope.  This produced spurious warnings with
the STATIC_ASSERT() macro (which used a typedef to provoke a compile
error in the event of an assertion failure).

This patch switches to a simpler technique that avoids the warning.

v2: Avoid GCC-specific syntax.  Also update p_compiler.h.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>