[722/906] gl: Fix build on Win32

1) Need to include GL/glext.h on W32 to provide necessary macros
2) W32 macros that pre-processor has by default are different for different
   toolchains. Borland uses __WIN32__, everyone else has _WIN32, so check both.

Side-note: glext.h is not supplied by mingw-w64 at the moment, but can be
downloaded from http://www.opengl.org/registry/

https://bugzilla.gnome.org/show_bug.cgi?id=703364

gst-libs/gst/gl/gstglapi.h

@@ -40,7 +40,7 @@
 # else
 #  include <GL/glu.h>
 #  include <GL/gl.h>
-#  if __WIN32__
+#  if __WIN32__ || _WIN32
 #   include <GL/glext.h>
 #  endif
 # endif