[CMake] External Projects: just what are they?

[Michael Powell]

Hello,

I am trying to understand external projects. What are they? What kind
of support is there for them?

I found: https://cmake.org/cmake/help/v3.9/module/ExternalProject.html#command:externalproject_add

For instance.

I have a Git Submodule which I want to support. It represents a core
library that is essential for a wrapper piece. If I understand
correctly, technically it is an "external" project, but is there
support for Git Submodules? And how does that work exactly?

Ideally, I'd like to include the Submodule CMakeLists.txt from the
context of my wrapper piece, but is this even possible?

Thanks!

Cheers,

Michael Powell