Portability

clFFT is no longer accepted on macOS

The Apple OpenCL compiler fails to produce a functional clFFT build, so configuring an OpenCL build with -DGMX_GPU_FFT_LIBRARY=clFFT on macOS is now rejected by CMake, rather than leaving PME on the GPU unavailable at run time. VkFFT, which is the default on macOS, should be used instead.