[CMake] Updating external source where URL is a directory?

[Tom Davis]

I am using ExternalProject_Add() to add a dependent project. When I'm developing
locally, I want to be able to make changes to the dependency and have them
pulled in by the parent build without having to commit said changes to a remote
repository. To include the dependency, I am using the `URL` parameter and
setting it to the absolute path to the other project.

This works fine initially, but re-making the parent project never updates the
dependency. Even cleaning the dependency doesn't work; the files in
$EP_BASE/Source/dep/ aren't updated so I end up re-making whatever outdated
source is there. What am I missing here?