scons: Support clang.

clang is supports most gcc options / extensions, with a some exceptions.

The biggest advantage of using clang is that compilation times are much
short.

One can tell scons to use clang when building by invoking it as

   CC=clang CXX=clang++ scons libgl-xlib

src/mesa/SConscript

@@ -351,7 +351,8 @@ get_hash_header = env.CodeGenerate(
 #
 # Assembly sources
 #
-if env['gcc'] and env['platform'] not in ('cygwin', 'darwin', 'windows', 'haiku'):
+if (env['gcc'] or env['clang']) and \
+   env['platform'] not in ('cygwin', 'darwin', 'windows', 'haiku'):
     if env['machine'] == 'x86':
         env.Append(CPPDEFINES = [
             'USE_X86_ASM',