Well, I'm testing the install right now! Pretty much what I did was get gnustep-startup-0.16.0 and gave it some options. I decided to use /usr/GNUstep as the root and will put symlinks on / (ie: /System -> /usr/GNUstep/System, etc). I looked through the ./InstallGNUstep script for some hints on how I could use gnustep-startup to send the files where I wanted. The script is fairly complicated and uses some options that are not shown on --help (like --install-root). I had to spend a few hours last night going through it and still don't understand a few parts, like where is the gs_build_package (to name a few) program that is run from inside the script.
These were the steps I took (I'm installing it to /tmp/gnustep-core):
I doubt it's going to work, but then again I'm not wasting anytime since I started the build before coming to work! I'll know how it went when I get there. If I can't get this to work the way I want to, I'll just build the packages separately instead of from gnustep-startup's script.
BTW, thanks for the info on libffi, I'll have to grab that!