ruby-install: rbx 2.3 fails to build on Yosemite due to llvm config error

As originally reported by @willcosgrove on https://github.com/postmodern/ruby-install/issues/181

~/src/rubinius-2.3.0/configure.log:

[2014-11-04 15:33:58] Configuring Rubinius...
[2014-11-04 15:33:58] Command line: "--prefix=/Users/swrobel/.rubies/rbx-2.3.0 --with-opt-dir=/usr/local/opt/openssl:/usr/local/opt/readline:/usr/local/opt/libyaml:/usr/local/opt/gdbm"
[2014-11-04 15:33:58] Checking gcc:
[2014-11-04 15:33:58]  found
[2014-11-04 15:33:58] Checking g++:
[2014-11-04 15:33:58]  found
[2014-11-04 15:33:58] Checking bison:
[2014-11-04 15:33:58]  found
[2014-11-04 15:33:58] ---

Detected old configuration settings, forcing a clean build

---
[2014-11-04 15:33:59]   Checking for 'llvm-config': 
[2014-11-04 15:33:59] not found
[2014-11-04 15:33:59]*** ERROR ABORT: unable to set up LLVM