scons: Whenever possible decide what to do based on platform and not compiler.

Because compilers like GCC and Clang are effectively available everywhere
so their presence/absence is seldom conclusive.

Furthermore, all compilers we use now have stdint.h.

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

src/mesa/SConscript

@@ -82,8 +82,7 @@ format_unpack = env.CodeGenerate(
 #
 # Assembly sources
 #
-if (env['gcc'] or env['clang']) and \
-   env['platform'] not in ('cygwin', 'darwin', 'windows', 'haiku'):
+if env['platform'] not in ('cygwin', 'darwin', 'windows', 'haiku'):
     if env['machine'] == 'x86':
         env.Append(CPPDEFINES = [
             'USE_X86_ASM',