Fix build problem on OS X Mountain Lion (10.8)

For about forever, our build flags for OS X have been slightly off, but
good enough to build and run. Apparently they aren't good enough any more.

Previously, we would compile with macosx-version-min unset and link with
it set. This combination, using GCC 4.8, on Mountain Lion, would create a
bad executable ("Illegal Instruction: 4", or something like that)

This patch consistently sets macosx-version-min for both compiling and
linking, which makes everything happy enough to build and run.


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@392279 f38db490-d61c-443f-a65b-d21fe96a405b