openmpi and libevent use

[Christopher Nielsen]

There’s an open ticket for OpenMPI, related to opportunistic use of libevent:

https://trac.macports.org/ticket/59626

It turns out that, as of OpenMPI v4.0, the cmake scripts now prefer the use of an external libevent, when available. Which means we end up with inconsistent behavior, depending on whether the user already has libevent installed.

This obviously needs to be fixed. The question is, do we want OpenMPI to build and use its own private libevent, or add a formal dependency on ours?

The developer docs don’t provide a strong recommendation either way:

https://www.open-mpi.org/faq/?category=building#libevent-or-hwloc-errors-when-linking-fortran

Given that the internal version was always built and used prior to v4.0, using that one might be a good choice. But I don’t have an opinion.

So I was hoping that you folks can help me make this call. Thoughts?