Macro for detection of boost MPI.

This pull request adds a function for detecting boost MPI, which uses BOOST_FIND_LIB.  If the MPICXX variable is set (by ax_mpi.m4, acx_mpi.m4 or similar), then set the CXX and CXXCPP variables prior to the check and restore them afterwards.  Otherwise the standard CXX variable is used.  I have tested this on OS X with macports boost +openmpi variant, as well as on Ubuntu with self-installed boost.  Hopefully others find this useful...