This is what we do with OpenFrameworks. There is an ofxAddonTemplate that is both documentation and a template when creating a new addon. We then provide makefiles and a ProjectGenerator app that work with this layout and, if the addon is in the right place, make it relatively easy to include the external sources and libs into your project. Now, pd externals wouldn’t require a project generator but would need a makefile that find the pd sources and builds against them and/or any other required libs. Again, we can borrow form the great OF makefiles in this regard.