boost: fix the location of mpi.so

1)
When compiling with python and mpi, the
python module mpi.so is not located in the proper place.
It should be in the python library dir, so that
>>> import boost.mpi
works, without e.g. altering the PYTHONPATH manually.

Added a simple command after install to move the mpi.so in the python libs.

2)
Commented the error message in the case python + mpi + c++11.
It works since 1.55.