This means that, even if box2d is installed afterwards, these activities won't be present.
On Debian side, it seems the qml-box2d library is not packaged (
https://github.com/qml-box2d/qml-box2d).
In GCompris, when it's not available we compile it ourselves (via -DQML_BOX2D_MODULE=submodule) but I'm not sure if it would be acceptable to
do it in Debian?
Best way would probably be for qml-box2d to be packaged, set it as a mandatory dependency and use -DQML_BOX2D_MODULE=system.
Note: I'm one of the GCompris developers, I don't know much about the packaging.
Johnny