Add 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.

Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>