CMakeLists.txt upgraded for Spack

[bjoo]

Setting MPI_XXX_COMPILERS from MPICXX, MPICC etc.environment variables breaks Spack based builds.All I have done here is if one adds -DQUDA_SPACK_BUILD to CMake it won't look at those env variables

[mathiaswagner]

Thanks. I guess the whole logic there is more a legacy thing from the last decade. Would hope we can remove all of this but never bothered so far.