Switch to MPI_lang_... variables

This MR switches the CMake configuration to use `MPI_<lang>_...` variables instead of the existing `MPI_...` variables. Currently Nektar++ uses `MPI_LIBRARY` and `MPI_EXTRA_LIBRARY`, which have been deprecated for some time. In recent versions of CMake, at least on OS X, `MPI_EXTRA_LIBRARY` is no longer set, meaning that Nektar++ fails to configure if MPI is enabled. A similar fix has also been applied to gsmpi which has the same issue.

See merge request !725