Some distributions place boost_mpi next to the MPI libraries against
which it was built instead of next to the other Boost libraries.  If
find_package(MPI) has already been run prior to find_package(Boost) then
MPI_CXX_LIBRARIES or MPI_C_LIBRARIES may be set to the location of the
MPI libraries.  Teach FindBoost.cmake to look there for boost_mpi and
boost_mpi_python after looking next to the other Boost libraries but
not consider the location to be Boost_LIBRARY_DIR.