Support for MinGW x64 compilers

So I've just compiled all the external dependencies for MinGW x64 compilers, adjusted the CMake file and tested them with for the following compiler builds:

* MinGW-w64 x64 rubenvb 4.7.2 release
* MinGW-w64 x64 rubenvb 4.7.1 experimental std thread
* MinGW-w64 x64 rubenvb 4.8 unstable
* MinGW-w64 x64 TDM

Everything seems to work across all the tested compilers.

Here's again a version list of the external libraries:

* OpenGL32: Converted the x64 VS import library
* libsndfile: Converted the x64 VS import library
* JPEG: 6b
* GLEW: 1.6.0
* FreeType: 2.4.4

If you need anything else, let me know!