Fix the build of OpenCV with XIMEA

I was not able to build OpenCV with XIMEA on Windows 64 bits using mingw:
- the "-fpermissive" compiler option (which has been added a few months ago) is a very good thing but the "cap_ximea.cpp" contained some awfull "goto" instructions
- the "CMAKE_CL_64" variable is not initialized when using mingw