Add defined(MPI_VERSION) check in h5py/api_compat.h.

Patch from @Arfrever for issue #439

Tested on both serial (hdf5 1.8.12, gcc 4.8, debian, python 2.7.5, without `--mpi`) and parallel (hdf5 1.8.13, openmpi 1.8.1, gcc 4.8, debian, python 2.7.5, with `--mpi`) hdf5 with `python setup.py test`.