egl: Use SConscript for Windows build.

Fix several portability issues and add SConscript for Windows build.
diff --git a/SConstruct b/SConstruct
index 1d41fc5..1c8a00c 100644
--- a/SConstruct
+++ b/SConstruct
@@ -49,7 +49,7 @@
 opts = Variables('config.py')
 common.AddOptions(opts)
 opts.Add(ListVariable('statetrackers', 'state trackers to build', default_statetrackers,
-                     ['mesa', 'python', 'xorg']))
+                     ['mesa', 'python', 'xorg', 'egl']))
 opts.Add(ListVariable('drivers', 'pipe drivers to build', default_drivers,
                      ['softpipe', 'failover', 'svga', 'i915', 'i965', 'trace', 'r300', 'r600', 'identity', 'llvmpipe', 'nouveau', 'nv50', 'nvfx']))
 opts.Add(ListVariable('winsys', 'winsys drivers to build', default_winsys,