You can build it once to install, say, in /usr/local/ruby-1.8, install
it (plus libraries/gems you may need) and then tar the whole directory.
Un-tar it in the same location on every new Mac. At least, this is what
I do, and not only on Macs. Somehow, any standard installation never
worked for me. Network file sharing may be used too. Even over NFS
everything worked sufficiently fast for me.