example:
https://github.com/YCAMInterlab/ofxTimeline/tree/master/ofxTimelineLib
Rick Barazza describes it at:
http://rbarraza.com/adding-ofxkinectforwindows2-to-a-new-or-existing-of-project/
(although he might have a couple of unnecessary steps there)
Essentially the pattern is:
* The addon has a .vcxproj which is added to the solution
* The addon has a .props which is added to your app project to give it any necessary settings
* Addon settings (e.g. what files to build, include paths, libs to link, dll's to copy, etc) are all handled by those two. Whenever any changes are made to the addon you don't need to apply settings to your projects manually
* You don't need to rebuild your addon per project which includes it (quicker build times esp. for large projects)
