Simplified uniform GPU selection in CMake

Simplified uniform GPU selection in CMake

GPU selection is now done by setting GMX_GPU to either CUDA
or OpenCL, with no other variables required. As part of the
overall CMake simplification, the CMake automatic detection
of hardware and "auto" settings for GPU acceleration have
been removed. This will require the user to explicitly enable
GPU support (which is a drawback), but it leads to much
simplier and shorter CMake code, similar support for CUDA and
OpenCL, and it will make it easier to handle multiple different
APIs targeting e.g. NVIDIA GPU hardware in the near future.