how to allow a port to choose an MPI library?

[Anton Shterenlikht]

I've submitted a port for lang/OpenCoarrays [1],
which at present depends on net/mpich.
I'd like to add options to allow the user to choose
the desired MPI library (port) from what is available at present:

net/mpich
net/mpich2
net/openmpi
net/openmpi2

How do I implement this correctly via port options,
with MPI_HOME etc. set appropriately?

Anton


[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=217283