Add fswatch dependency in Savi compiler, for watching file changes.

[jemc]

I intend to add a --watch flag to the Savi compiler, but first I need a cross-platform library for watching file changes.

This PR intends to add that library.

[jemc]

@mneumann - when you get a chance, can you please try this locally on your FreeBSD machine?

We don't yet have a working FreeBSD CI setup, so I can't be sure that I'm not breaking the build for you on FreeBSD.