Use openmp on osx

[isuruf]

Checklist

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

[conda-forge-linter]

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

I wanted to let you know that I linted all conda-recipes in your PR (recipe) and found some lint.

Here's what I've got...

For recipe:

• Failed to even lint the recipe, probably because of a conda-smithy bug 😢. This likely indicates a problem in your meta.yaml, though. To get a traceback to help figure out what's going on, install conda-smithy and run conda smithy recipe-lint . from the recipe directory.

[isuruf]

We are using llvm-openmp with both gfortran 7.3 and clang 9.0 and llvm-openmp is fork safe.

[beckermr]

We should link that issue from the main issue tracker where this was discussed.

[beckermr]

found it: conda-forge/conda-forge.github.io#831

[isuruf]

Merging as there was no objection.