On Macs with Python 2.6, we are having issues building MPI4PY. We get the
following error:

Undefined symbols for architecture x86_64:
  "__PyByteArray_empty_string", referenced from:
        _PyMPI_Allocate in MPI.c.o

For now, disabling building of MPI4PY in Mac binaries.

Change-Id: Ic4f783624658d8e9874c06efdffc19c7711c1535