CGAL ipelets

This pull request adds cgal-ipelets formula (ipelets which are included with CGAL, but not installed by default). It also fixes a small, but inconvenient bug in the ipe formula. 
Sorry. I think I misunderstood the question. The answer is "yes." It will work. The `IPEPREFIX=...` in `make install` is only used for determining where to copy the files.