clang either does not build or defectively builds certain things on OS X, for instance Ruby 1.9.3. I had to acquire vanilla GCC for this reason the other day, and was relieved to find it in HomeBrew.
brew install rbenv ruby-build; /* rc file shenanigans */; rbenv install 1.9.3-p327; rbenv global 1.9.3-p327; ruby --version