In order to build this as a module, clone Godot repo:
git clone https://github.com/godotengine/godot.gitClone this repo in the modules folder as rapier_3d:
git clone https://github.com/appsinacup/godot-rapier-physics-module-3d.git modules/rapier_3dBuild as normal, eg.:
sconsWorking example: Gonuts Repo.