[CMake] How to have a target depend on an External Project

[kent williams]

For better or worse (mostly better) we are now heavy users of
ExternalProject.  That module works really well to pull in external
dependencies and get them built.

ExternalProject_add has a DEPENDS keyword that lets you specify
dependencies on other External Projects.  But an ExternalProject isn't
an actual CMake target, so I can't figure out how to make a regular
CMake target depend on an External Project.

Suggestions?