am a681bae5: am 9a8732fb: Link against Mac OSX 10.5 SDK if building on 10.6.

Merge commit 'a681bae5f6d2cc84d0ff54bb117ecd025062bbce'

* commit 'a681bae5f6d2cc84d0ff54bb117ecd025062bbce':
  Link against Mac OSX 10.5 SDK if building on 10.6.