[CMake] external projects and transitive dependencies

[Brian Lewis]

If X brings in Y as an external project, and Y brings in Z as an
external project, is there some way for Y to communicate information
about Z back to X, e.g., some include directory?

My situation is, X doesn't know anything about Z, but needs to be able
to find one of Z's headers. I wish I could add Y as an external project
and then include_directories(${Y_INCLUDE_DIRS}).