mpi PG should be fixed: currently mpi.setup require demands for mpich-default which demands clang

[barracuda156]

Example of the problem:

10:~ svacchanda$ /usr/bin/sudo port -v -n build parmetis
--->  Computing dependencies for parmetis..mpich-default is known to fail. Try to install anyway? [y/N]: n
Error: mpich-default is known to fail
Error: Follow https://guide.macports.org/#project.tickets if you believe there
is a bug.
Error: Processing of port parmetis failed

Now, the only combination that allows the build to proceed is this:

if {${os.platform} eq "darwin" && ${os.arch} eq "powerpc"} {
    mpi.setup       require require_fortran
} else {
    mpi.setup       require
}

Notice, require -clang -fortran does not work. Even fortran is not needed at all, require_fortran is needed to trick around mpi PG stupidity.