I'm still not convinced there's a good way in cmake to pull in and compile external libraries as dependencies. I know it can be done via a superproject method but that's a lot of boilerplate for something that should be straightforward. Everything else is simple and easy until I butt heads with that issue.
It's been a while since I've done this kind of CMake in anger, but I recall my main gripe with ExternalProject_Add is that it can't leverage that the external project is a CMake project and I still end up hard coding paths.
function (AddDependency name)
add_subdirectory(${PROJECT_SOURCE_DIR}/deps/${name})
include_directories(${PROJECT_SOURCE_DIR}/deps/${name}/include)
endfunction (AddDependency)
And then use git submodule to clone all the dependencies I need in the deps folder. Everything then works automagically (assuming that the dependency is built through CMake).
I wrote up a little thing about all the alternatives https://geokon-gh.github.io/hunterintro.html. Tldr: Hunter is by far the best options and makes CMake painless. It also solves some multiple-dependency and toolchain issues that are nearly impossible to get right with vanilla CMake
