FindMPI: Correct legacy variable handling
Due to a minor logic error, MPI_LIBRARY, MPI_EXTRA_LIBRARY and MPI__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)