am 9a8732fb: Link against Mac OSX 10.5 SDK if building on 10.6.

Merge commit '9a8732fb6c6a569495e3695fc1e2b638de209beb' into gingerbread-plus-aosp

* commit '9a8732fb6c6a569495e3695fc1e2b638de209beb':
  Link against Mac OSX 10.5 SDK if building on 10.6.