This is just a demonstration plugin that shows how to create a bare minimum filter plugin for use with the mjpg-streamer input_opencv plugin.
To create your own filter plugin, just copy filter_cpp.cpp to your own project, and compile/link it with the same build of OpenCV that mjpg-streamer is linked to.
CMakeLists.txt is specific to the mjpg-streamer build tree, and won't be useful outside of it.