Due to a minor logic error, MPI_LIBRARY, MPI_EXTRA_LIBRARY and
MPI_<LANG>_LIBRARIES hints are currently not correctly handled and
discarded.

Additionally, if either C or CXX aren't enabled, there subsequent calls
to FindMPI will cause cache variables of the sort "MPI_MPI_EXTRA_LIBRARY-NOTFOUND"
to appear due to an error in the compatibility handling. (Doesn't affect
functionality)