use -fopenmp instead of -openmp for Intel compilers, Intel's mpif90 only reacts to -fopenmp (+ style fixes)

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` (!)...