It is apparently for a piece of code that no longer exist.
There don't seem to be any part of the code referring to this directory (anymore, if there was).
- drop old build system
- move shared include and source files to common directory
- drop "rc-" prefix from shared include and source files
- move executable-specific code to individual directories under src
- adjust top-level .gitignore file for new build system
This closes#489.