It looks like the default behavior for scanning the folders has been overridden by the removal of the comments (e.g. # ADDON_INCLUDES =). So Project Generator does not include any of the required files in the subfolders. This is easily fixed by adding the comments back as I've done in my fork https://github.com/Quasimondo/ofxTimeline/blob/master/addon_config.mk but since I made other changes there I don't want to bother you with a pull request.
It looks like the default behavior for scanning the folders has been overridden by the removal of the comments (e.g. # ADDON_INCLUDES =). So Project Generator does not include any of the required files in the subfolders. This is easily fixed by adding the comments back as I've done in my fork https://github.com/Quasimondo/ofxTimeline/blob/master/addon_config.mk but since I made other changes there I don't want to bother you with a pull request.