Declaring a dependency on a system package that is not available on Windows

edit

rosdep isn't available on Windows. Instead you need to manually install dependencies. So if you don't have a certain dependency available but the CMake logic works around this gracefully then the package will build just fine. There is no logic during colcon build which will test if dependencies declared in the manifest are present or not.

On the other hand if you would want to declare a dependency which would be possible to provide on Windows the current CI builds on ci.ros2.org would at first fail until we update the Dockerfile used on Windows to actually install that dependency.