Write forward declarations in another way to avoid
repeated typedefs "error: redefinition of typedef".
Raised when using i686-apple-darwin11-llvm-gcc-4.2
It seems that C apple compiler does not support
C11 feature.
https://bugzilla.gnome.org/show_bug.cgi?id=703885
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
