Make the Makefile logic more explicit and move the Snow Leopard logic down to...

Make the Makefile logic more explicit and move the Snow Leopard logic down to where it's not executed on non-Darwin systems.

(closes issue #17028)
 Reported by: pabelanger
 Patches: 
       issue17028_20100315.patch uploaded by seanbright (license 71)
       20100315__issue17028.diff.txt uploaded by tilghman (license 14)
 Tested by: tilghman, pabelanger


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@252488 65c4cc65-6c06-0410-ace0-fbb531ad65f3