MacOS support

[dcbaker]

All current work has been done on Linux. MacOS support shouldn't be too hard, given that it is still a Unix-like OS.

Although we could likely use homebrew to install the dependencies, I think it would be better to not do that, since that will give us a test that Meson builds all of it's own dependencies without a chance of accidentally getting headers from /usr/include, which helps to catch cases were headers are needed but explicitly given

[ChrisThrasher]

I was able to build this on macOS and run the tests successfully without modification. What else is required to fully support macOS?

[dcbaker]

A CI job would be nice.

#31 doesn't implement the Windows or MacOS paths, just the *nix ones, so that would need to be added. I'm starting to rebase that now

[bretbrownjr]

We'll need to get someone to sponsor CI or we'll have to be judicious about our CI fan-out on every commit, but I agree that we need automated testing.

Seems like running a weekly job for every supported configuration is the minimum requirement.

[dcbaker]

I do have #52, which should help with CI usage