Not sure if this can help you, but I simply use

  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).