With a little help, I managed to compile cairo on my OS X such that irb
can include cairo without error message. So now, I get the same error
message as Farrel

I checked on rb_cairo.h, it is in
/private/opt/local/lib/ruby/site_ruby/1.8/powerpc-darwin8.3.0/rb_cairo.h

The person who helped me with my cairo installed thought that the
problem was with my having an old gnome2, but the darwinport folder
said ruby-gnome2-all-0.15.0. he thought 0.15.0 was alright.

he suggested getting help at
http://lists.sourceforge.net/mailman/listinfo/ruby-gnome2-devel-en

I found the ruby-gnome2-all-0.15.0/gtk/mkmf.log, I posted it at
visionlab.harvard.edu/Members/Anne/mkmf.log