You can a system wide install, with:
sudo make install
Or, if you would like to build the plugin statically into your project, then you put a copy under your own project (say, /qml-box2d), add the necessary includes and registration calls to your sources and build files.
Add somewhere in your .pro file:
And in your main.cpp includes, add:
And before using the qmldeclarativeview, register box2d plugin