Up the minimum OS X version to 10.6.

 * This allows us to remove some special-case build logic.
 * 10.5 is down to less that 8% of the OS X market share. 10.4 is down to 
   under 2%.
 * Apple is no longer releasing security updates for 10.5 and earlier.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@379495 65c4cc65-6c06-0410-ace0-fbb531ad65f3