Fix the build of OpenCV with XIMEA on Windows 64 bits (PR#1039 modified to be able to merge on branch 2.4)

Fix the build of OpenCV with XIMEA on Windows 64 bits: 
  - crosses initializations in "cap_ximea.cpp" (which also contained some awfull "goto" instructions)
  - the "CMAKE_CL_64" variable is not initialized when using mingw

PR#1039 modified to be able to merge on branch 2.4