When you (or Apple, in this case) configure and build Ruby, Ruby
records the location of its header files. If you later try to build an
extension, extconf.rb expects to find those header files in the same
place. However, Apple in its wisdom chose not to put them there.

Some folks have been satisfied to create symlinks for the header files
in /usr/lib/ruby/1.8/powerpc-darwin8.0, which works. Most folks, I
think, are simply building their own Ruby. It's not hard or
time-consuming.