Fixed an error with type MPI_Message missing for older versions of MPI and

newer versions of mpi4py.

pymusic/mpi_compat.h

+#include <mpi.h>
+
+#if (MPI_VERSION < 3) && !defined(PyMPI_HAVE_MPI_Message)
+typedef void *PyMPI_MPI_Message;
+#define MPI_Message PyMPI_MPI_Message
+#endif

pymusic/pybuffer.pxd

+# The following include is needed to define a missing type MPI_Message
+# which is lacking for a certain combination of MPI and mpi4py versions
+cdef extern from "mpi_compat.h":
+    pass
+
 cimport mpi4py.MPI as MPI
 from mpi4py.mpi_c cimport *
 

pymusic/pymusic.pxd

+# The following include is needed to define a missing type MPI_Message
+# which is lacking for a certain combination of MPI and mpi4py versions
+cdef extern from "mpi_compat.h":
+    pass
+
 cimport mpi4py.MPI as MPI
 from mpi4py.mpi_c cimport *