Fixing Cmake for the OpenCL build. 

Still in progress, will not work right now in a machine without CUDA.
OpenCL detection only implemented for Linux for now.
Better use -DGMX_OPENCL_FORCE_CL11_API=on or -DGMX_OPENCL_FORCE_LOCAL_HEADERS=on
along with -DGMX_USE_OPENCL=on and -DGMX_GPU=on