I'm using Pencil right now to map out an upcoming project. It's one of the few useful GUI prototyping apps I've found that's free. Most of the competition is for-pay, and I just don't do enough of this stuff to justify paying for squares and arrows on a PNG (I use it for exporting UML and page flows ATM.)

If you don't want to build it, just use the original project's pre-built installation (RPM, Bundle, EXE here http://pencil.evolus.vn/Downloads.html). The difference between this reboot project and the original is likely minimal, so if you like the original, it's probably worth coming back and building the reboot project.