scons: Use -std=gnu99

It a scary world out there: people use all sort of non standard C stuff,
and we must enable support for that in here in order to build.

-pedantic still warn us when we use that nonstandard though.

scons/gallium.py

@@ -313,7 +313,7 @@ def generate(env):
             '-Wmissing-prototypes',
             '-Wno-long-long',
             '-ffast-math',
-            '-std=c99',
+            '-std=gnu99',
             '-pedantic',
             '-fmessage-length=0', # be nice to Eclipse
         ]