Allow extensions to trigger integration builds of downstream extensions

[krame505]

For example, ableC-refcount-closure depends on ableC-closure, and so whenever a change is made to ableC-closure we should also build ableC-refcount-closure.

One potential issue: extensions that depend on multiple other extensions shouldn't get rebuilt more than once when the dependencies are triggered by ableC. Maybe we could add a build parameter to extensions of whether to build downstream extensions - an ableC build would want to trigger every known extension individually, but building any extension directly should default to also building any downstream extensions.