Just an FYI for those trying to build Ruby 1.8.6 on Leopard, found this thread which linked to the following patch:

http://chopine.be/lrz/ruby-osx-patches/ignore-gsetcontext.diff

Apply it to Ruby 1.8.6-p111 source and off you go. What this means, primarily, downloading that file, copying it to the directory where you have the Ruby source code, and then running patch -i ignore-gsetcontext.diff. That should take care of it for you. Then just build/run as usual.

BTW, loving Leopard. Things feel snappier.