scons: Build ws_xlib on Mac OS X.
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 <25a259f709b0f1b620605d2518271a846d8826e5@freedesktop.org>
Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

bkaradzic/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer,mcanthony/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,wolf96/glsl-optimizer,zeux/glsl-optimizer,dellis1972/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,metora/MesaGLSLCompiler,zz85/glsl-optimizer,metora/MesaGLSLCompiler,tokyovigilante/glsl-optimizer,zeux/glsl-optimizer,mapbox/glsl-optimizer,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,dellis1972/glsl-optimizer,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer,djreep81/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,benaadams/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,metora/MesaGLSLCompiler,mapbox/glsl-optimizer,wolf96/glsl-optimizer,zz85/glsl-optimizer,mcanthony/glsl-optimizer,mapbox/glsl-optimizer,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer,mapbox/glsl-optimizer,zeux/glsl-optimizer,djreep81/glsl-optimizer,jbarczak/glsl-optimizer,jbarczak/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,tokyovigilante/glsl-optimizer,tokyovigilante/glsl-optimizer