ENH add external build for linking against system MPI libs

[beckermr]

This PR adds a dummy package with lower solver priority to allow for users at HPC systems to link their packages in conda compiled against openmpi to the system openmpi libraries. This is a requirement for most HPC systems. The other major mpi distribution in conda-forge, mpich already supports this. See PR https://github.com/conda-forge/mpich-feedstock/pull/44.

We'll need to change the docs which mentions this for mpich here: https://conda-forge.org/docs/user/tipsandtricks.html#using-external-message-passing-interface-mpi-libraries

Closes #90

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [x] Reset the build number to 0 (if the version changed)
• [x] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [x] Ensured the license file is being packaged.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[beckermr]

@conda-forge-admin rerender

[minrk]

Is it possible to get a skip so it isn't rebuilding the same empty package on all platform combinations?

[beckermr]

The empty package is not the same since it is on different platforms. I worry the possible difference in solver priority between noarch and platforms would mess with track features. I don't think this is a good idea.

[minrk]

Fair enough. I think using noarch: generic would be the right way to go for the empty package.

[beckermr]

Thanks @minrk. Sorry for the quick merge here. If you’d like to be using noarch here, we can pick that up in a new PR.

[minrk]

Not a big deal! The build matrix is just getting big, especially with cuda compilers now. I might have a go the next time I'm bothered waiting for some builds to complete, but waiting-for-build time is the only real cost.