fixed gl.h compile time errors in VS2013 (SFML2 sample)

according to http://stackoverflow.com/questions/430413/vc-compile-errors-when-including-gl-h the gl.h errors are specific to VC++, there placing a condition there elimates the for VC++ people to uncomment

    #include <windows.h>
A couple of notes for this one.  In general you would probably want to use _WIN32 which covers pretty much all Windows compilers, where as _MSC_VER is specific to only visual studio.

In the case of libRocket specifically, we use ROCKET_PLATFORM_WIN32 in place of both