--with-mpi-include= and --with-mpi-lib= are not respected

When we are trying to use openmpi using these config vars:
```
CONFIGURE_ARGS+=        --enable-mpi \
                        --with-mpi=${MPI_PREFIX} \
                        --with-mpi-lib="${MPI_LIBS}" \
                        --with-mpi-include="${MPI_INCLUDES}"
```

these are not respected and it still uses mpich instead of openmpi values.

