As mentioned above, there's a whole bunch of methods that are all better than abusing the preprocessor for this.
a) If stuff is versioned, why not use a version control system instead of folder names?
b) Failing that, the -I option exists for exactly that kind of thing (ie. it belongs in your project settings or Makefile, not the actual source code)
c) Use a symlink that points to whatever version you want to use (yes, Windows finally has them too)