Preloading a project in Cmake?

Question

CMake

My project uses ChibiOS RTOS and Cmake as builder. But I can’t quite understand how to check the presence of this library in Cmake (it will just go to check if the folder exists or not) and if it doesn’t exist, then pull a specific branch or commit from git / svn.
I found ExternalProject_Add, but Chibios is not built as a library - it needs to be compiled along with the project files. But as far as I understand, ExternalProject_Add just adds a dependency for the Makefile, and Chibios must be downloaded before add_executable, so that I can specify which files from Chibios to compile.
So far, I can't find how to do it.