The Ruby that ships with Mac OS X 10.4 (Tiger) is pleasantly current but has an issue that prevents C extensions from compiling and installing properly. In case you’ve just upgraded and want to use the Apple-supplied Ruby, go install RubyGems (as if you haven’t already!) and do:
        sudo gem install fixrbconfig
        sudo fixrbconfig

This will fix some settings in rbconfig.rb that you probably don’t care to fix by hand.