Intel fftw mpi update for MKL >= 11.1

this is specific to imkl v11.1 and up, so if should only be defined in that case (i.e. in the `else` below)

also: I feel lowercase suffixes make more sense, and this can be shortened to:

```python
mpi_suff = ''
for mpilib in ['MPICH2', 'MPVAPICH2', 'OpenMPI']:
    if get_software_root(mpilib):
        mpi_suff = '_%s' % mpilib.lower()
        break
```

Should we also include `impi`? Right now, there will be no suffix when `impi` is used, which is bad (it will overwrite the non-MPI lib)?