scons: Build ws_xlib on Mac OS X.

Fixes this SCons build error on Mac OS X if X11 is found.

NameError: name 'ws_xlib' is not defined:
  File "SConstruct", line 144:
    duplicate = 0 # http://www.scons.org/doc/0.97/HTML/scons-user/x2261.html
  File "scons-2.2.0/SCons/Script/SConscript.py", line 614:
    return method(*args, **kw)
  File "scons-2.2.0/SCons/Script/SConscript.py", line 551:
    return _SConscript(self.fs, *files, **subst_kw)
  File "scons-2.2.0/SCons/Script/SConscript.py", line 260:
    exec _file_ in call_stack[-1].globals
  File "src/SConscript", line 34:
    SConscript('gallium/SConscript')
  File "scons-2.2.0/SCons/Script/SConscript.py", line 614:
    return method(*args, **kw)
  File "scons-2.2.0/SCons/Script/SConscript.py", line 551:
    return _SConscript(self.fs, *files, **subst_kw)
  File "scons-2.2.0/SCons/Script/SConscript.py", line 260:
    exec _file_ in call_stack[-1].globals
  File "src/gallium/SConscript", line 135:
    'targets/libgl-xlib/SConscript',
  File "scons-2.2.0/SCons/Script/SConscript.py", line 614:
    return method(*args, **kw)
  File "scons-2.2.0/SCons/Script/SConscript.py", line 551:
    return _SConscript(self.fs, *files, **subst_kw)
  File "scons-2.2.0/SCons/Script/SConscript.py", line 260:
    exec _file_ in call_stack[-1].globals
  File "src/gallium/targets/graw-xlib/SConscript", line 9:
    ws_xlib,

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/winsys/sw/xlib/SConscript

@@ -4,7 +4,7 @@
 
 Import('*')
 
-if env['platform'] in ('cygwin', 'freebsd', 'linux', 'sunos'):
+if env['platform'] in ('cygwin', 'darwin', 'freebsd', 'linux', 'sunos'):
 
     env = env.Clone()