Build on Linux and Windows as part of GitHub Actions

[JostMigenda]

In #4, I’ve focussed on getting this to build on macOS; and the workflow is currently skipping Linux and Windows which I can’t test on my machine. @jpkneller and @Sheshuk, could you help with this?

Ideally, this would only require modifying a single install step per OS, to

1. install any OS-specific dependencies
2. export the LIBOMP_INCLUDE environment variable

… and everything else should remain unchanged.
(In practice, I suspect it won’t go quite as smoothly; but we should be able to figure this out.)