I'm surprised not to see generic tools on this list which make use of event-based file system notifications such as dnotify and inotify. See https://github.com/rvoicilas/inotify-tools/wiki or inotifywait (https://superuser.com/a/959040) for examples.

I know this particular API (inotify) is specific to Linux (there might be similiar APIs on other OS). But it is awesome for all kind of build systems, for instance latex documents.