[Discussion] What strategy do you use to have a project build on CircleCI when an external project is updated?

If you have a project that you build on CircleCI, it will typically build when you push up a commit or if a fork PR is opened.

If this project in someway depends on another project, how do you manage this relationship? How do you kick off a build for your project when the dependent project is updated?

Lastly, how does your answer change if that dependent project isn’t one you own?

I consider “updated” here to mean any of, a new commit occurs on master, or maybe a git tag is pushed.