Fixed wrong conversion from pickled bytearray to str in MessageCallback. The pickle protocol adds some meta information bytes in front of the actual string, so a conversion to Cython str is invalid in general.

Version 2 of mpi4py has changed the name of a submodule from mpi_c to
libmpi. We need to check which one to use at compile time.

newer versions of mpi4py.

not released version
SVN: multiconn@1048