Adding MPI back to Boost

[Sean Farley]

So, I've added support into the compilers portgroup for working with
the compiler.blacklist variable but that doesn't fix the issue that Ryan
tried to fix with r125939:

https://trac.macports.org/changeset/125939

It seems instead that the fix is just to remove the gcc variants:

mpi.setup -gcc

Is there anything wrong with this proposed fix? If no one objects, I'll
push my changesets which 1) add blacklist support to the compilers
portgroup and 2) add mpi portgroup to boost (but remove gcc variants).