Get threaded version working again

[thomasballinger]

For maintainability (plus if this were every to get upstreamed to https://github.com/endless-sky/endless-sky) the diff should be smaller. Let's get threads working again, even if it is a worse experience. Now that Safari supports it it should be fine!

Previously a threaded build was working: https://github.com/endless-sky/endless-sky/pull/6164

The goal is that the first two commits of the endless-web branch should be enough to have a working build using threads, but also that a threaded version work at the tip of endless-web.