enable -mt_mpi compiler flag is both usempi and openmp toolchain options are enabled

This PR mainly changes the use of `-openmp` to using `-fopenmp` instead for Intel compilers.

Strictly speaking, both are correct, but apparently the `mpif90` wrapper that comes with Intel MPI only reacts to `-fopenmp` to enable linking with `-lmpi_mt` (the thread-safe MPI library); it doesn't react to using `-openmp` (!)...