I have only once attempted to build it on FreeBSD. I started by installing dependencies, and then ran out of time and stopped before I finished getting the system set up to do this.
Unfortunately this is related to FLTK dependencies, and Rakarrack does not use XOpenDisplay() directly.
One thought is to comment this check out of the configure script and see if you can build without it and cross your fingers that your FLTK and X11 versions work this out in a different way. If it is a success, then the configure script apparently is checking for something which may be implemented slightly differently on FreeBSD, but still compatible between FLTK and X11.
Let me know if that hack has any success. If make fails, perhaps you can report where make fails and what file.