The docs for LLVM don't seem to be that great when it comes to compiling this stuff from scratch, so here's what I did, as one big script chunk. You can probably copy and paste this, but I make no promises that it will work, only that it Works on My Machine.
And that should output something like
The last part about the LLVM build is the important part. It still doesn't quite work with llvmruby, but maybe it's because I used the 2.5 version and the llvmruby page says 2.4. Just something else…
LLVM is up and running though, so happy LLVM'ing!