This was used by Rubinius over the years to ensure that a correct llvm version was selected from the ones available on the system, but for recent Rubinius this function is a no-op, and since Rubinius isn't an actively maintained project anymore, I don't feel strongly for keeping ruby-build workarounds that only help ancient Rubinius versions.
Rubinius fails to download a prebuilt LLVM on Yosemite since one is not
available yet. Instead, download the prebuilt version for the previous
OS X release.
This fixes Rubinius 2.2.7+ builds, but the older ones still fail for me
on Yosemite. This could be due to the fact that they're old releases
which are not compatible with never dependencies on the system.
Rubinius never supported the `--with-libyaml-dir` configure option, so
we can never use `--if needs_yaml` since it's incompatible and might
break the build if libyaml was auto-discovered from Homebrew.
Starting from 2.2.0, Rubinius doesn't even need libyaml anymore since it
stopped building psych. Instead it requires users to install psych as a
gem which vendors its own libyaml anyway.
