Although the non-standard GCC syntax has some nice properties, for most
practical cases the standard C99 syntax is perfectly fine.  Particuarly
for printf-like macros, which pretty much account for most the uses of
variadic macros in piglit.

Unfortunately this will only be effective on newer GCC versions, due a
bug in GCC.  See comment for more details.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>