Added `--without-mpi` for `link=static`.

* boost/jamfile: Disabled the build of `libboost_mpi.*` for the case of
  `link=static` since it depends on the shared variants of
  `libboost_mpi_python.*`, `libboost_python.*` and
  `libboost_serialization.*`.
Merge branch 'master' into fix/boost_mpi
Merge pull request #55 from Cryolite/fix/boost_mpi

Added `--without-mpi` for `link=static`.