Add conditional definition of PyMPI_MPI_Message and MPI_Message PyMPI_MPI_Message. This is needed for MPI_VERSION<3 and a recent version of mpi4py.

Follows gh-401 https://groups.google.com/d/topic/h5py/Uw5x3BKwJGU/discussion and https://groups.google.com/d/topic/mpi4py/xnDyYvawB-Q/discussion.
Merge pull request #404 from pdebuyl/mpi_message_header

Add conditional definition of PyMPI_MPI_Message and MPI_Message PyMPI_MP...