egl: Use SConscript for Windows build.

Fix several portability issues and add SConscript for Windows build.
diff --git a/src/SConscript b/src/SConscript
index b72491d..b8a0024 100644
--- a/src/SConscript
+++ b/src/SConscript
@@ -3,6 +3,9 @@
 SConscript('glsl/SConscript')
 SConscript('mapi/glapi/SConscript')
 
+if 'egl' in env['statetrackers']:
+    SConscript('egl/main/SConscript')
+
 if 'mesa' in env['statetrackers']:
     SConscript('mesa/SConscript')