project setup with usage of external project


my project consists of nearly 20 subprojects and all use the same Qt- and
other packages and they use “that” external project.

The external project is selfcompiled from github, but I don’t want to import
build instructions into my project. Just use it as is.
That foreign project is not built by cmake, so I have to code references on my
From the external project I use several include directories and several libs
(static libs and shared libs as well), so I think, find_library is not best

I wonder how could I write the dependencies minimizing code duplication.
Should I consider using an include-file, that contains cmake statements like
CMakeLists.txt or does it make sense to code a module?

If the latter is true, is there a tutorial/howto about writing a module?
Did I get it right, that its not possible to access variable definitions of a
parent directory? So I guess having subdirectories in different depth are in
favor of module definition. Right?