Now, it surely isn't as bad as I've just made it out to be -- surely people have automated this? But as far as I can tell that process is not documented anywhere, and every popular Ruby project I've checked has its own complicated Rakefile that requires even more software installs to make this stuff work.
It occurs to me now that the simpler approach is to just tell users "download this file and stick it in your directory if you wanna use it". But that isn't sufficient for more complicated, multi-file stuff like ljrb.
Mysteriously, the only programming language that seems to have solved this well is C: if I need a C library I apt-get it, and everything else uses autoconf so the build flags are the same between projects. Don't get me wrong -- autoconf is certainly agonizing but is a single point of pain. (I wish gtk-doc wasn't such a pain to use, too.)