Add more complex cmake example to CI

[ChillerDragon]

Using ddnet_base in a lib and using that lib in a ddnet client was a bit of a mess. I described my adventures here chillerbot/chillerbot_reply#2

This case should be checked in the CI in this repo. So the CMakeLists.txt can safely be edited without breaking projects that use it.
And there is a ready to use working example cmake for consumers that is known to be working and checked in the CI.