Fix for MinGW-w64 compiler

Third attempt :)

Documentation on GWLP_WNDPROC and LONG_PTR:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms644898%28v=vs.85%29.aspx
Basically they're polymorphic for 32bit and 64bit applications.

Also, there's no `__MINGW32_VERSION` macro in MinGW-w64 and openFrameworks uses `__MINGW32__` everywhere.