progs: Include X11 headers and libraries in SCons build.

Fixes Mac OS X SCons build.
diff --git a/progs/SConscript b/progs/SConscript
index a90e13b..4cd35fe 100644
--- a/progs/SConscript
+++ b/progs/SConscript
@@ -24,6 +24,7 @@
 if progs_env['platform'] == 'windows':
     progs_env.Prepend(LIBS = ['glu32', 'opengl32'])
 else:
+    progs_env.Tool('x11')
     progs_env.Prepend(LIBS = ['GLU', 'GL'])
 
 # Glut