scons: Do not build glx and egl on Cygwin.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: José Fonseca <jfonseca@vmware.com>
diff --git a/src/SConscript b/src/SConscript
index 777ad23..e4cda2f 100644
--- a/src/SConscript
+++ b/src/SConscript
@@ -22,7 +22,7 @@
 SConscript('mapi/vgapi/SConscript')
 
 if not env['embedded']:
-    if env['platform'] not in ['windows', 'darwin', 'haiku']:
+    if env['platform'] not in ('cygwin', 'darwin', 'haiku', 'windows'):
         SConscript('glx/SConscript')
     if env['platform'] not in ['darwin', 'haiku', 'sunos']:
         SConscript('egl/main/SConscript')