Add macOS builds to CI

[ryandesign]

Looking through the ChangeLog and NEWS files, I saw a few cases where new releases had to be made because the previous one would not build on macOS, and I am not able to build the current version on macOS. The occurrence of releasing versions that cannot be built on macOS might be reduced by adding macOS builds to your continuous integration. I plan to send a PR to do this, after I can get it building on my local machine, so this issue is just a reminder to myself and also a request for comments.