It seems to missing a copy of pkg-config.rb so I've mailed the cairo list to see where they want me to get it from. I'll commit it once they say.
I have one pkg-config.rb in /usr/lib/ruby/site_ruby/1.8/pkg-config.rb (package dev-ruby/ruby-glib2-0.16.0). Not sure if that's what you were looking for.. Ahh.. the diff between that pkg-config.rb and rcairo-1.2.0's seems subtle
Version 1.4.1 is now in the tree and includes it's own pkg-config.rb