science/gromacs: Multiple improvements

* remove dependency on xorg: last xorg utility was retired some time ago
* add HWLOC option and pass cmake argument: hwloc2 is only used when
  this argument is supplied
* refactor OPENCL option: add OPENCL_NONE, OPENCL_ICD, OPENCL_SYCL options
* refactor SIMD option: use the AUTO value when set
* correct OPENMP option