Added parameters to config.darwin so that development can occur on 64-bit...

Added parameters to config.darwin so that development can occur on 64-bit architectures with i386 version of ISIS;  also added parameter such that MACOSX 10.5 is the minimum deployment version targeted.  These changes provide support for ISIS development on MACOSX 10.6 systems with exiting builds.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2792 41f8697f-d340-4b68-9986-7bafba869bb8