Fix readlink for os x.
Fix readlink for os x.

This should fix #104.

snowleopard/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,snowleopard/hadrian,izgzhen/hadrian,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,quchen/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,bgamari/shaking-up-ghc