Support for OpenCV 2.4.8 compliation under Mingw32-w64 

This pull request
* Updates opencv to 2.4.8
* Allows the package to be built under mingw32-w64

Previously this was not possible due to some missing types in mingw32-w64 float headers see https://bugzilla.redhat.com/show_bug.cgi?id=843436

The pull request is pretty sloppy but I've already spent a large amount of time getting my cross compilation working and I'm leaving this here incase someone else needs opencv support under mingw32-w64.
It breaks the mingw32 build which should be fixed and probably needs some cleaning up before being accepted into the mainline.

Thanks for all your hard work - mxe has made cross compilation for windows much easier!