Add configure command-line argument to indicated that MPICH is being built

Output Effect

Adds the command-line switch --with-mpichbuilding that may be used to indicate that MPICH is building. This allows a configure to work-around the fact that during a build of MPICH, certain commands, particularly the compilation commands such as mpicc, are not yet functional. The variable pac_lib_mpi_is_building is set to yes if in an MPICH build, no otherwise.

See Also