[cmake builder] Add install setting

[TheAssassin]

While trying to use appimagecraft on OpenSCAD, I noticed that this project does not provide any make install functionality.

Of course it'll be best to patch OpenSCAD (I hope I can send a PR soon), but appimagecraft should not insist on make install, since e.g., with linuxdeploy, a working AppDir can also be created by hand using -e, -d, i et cetera.