Makefile improvements for compiling on OS X

Add a Mac SSE4.2 target. Also change the Mac OS X minimum version to
10.6. Rationale: 97% of Macs run at least 10.6, version 10.9 is now
free, and using 10.6 as the minimum version gives a small 5% boost in
benchmark speed over versions using 10.0 as the minimum version.
Finally, enable Clang’s Link Time Optimization when compiling for the
Mac.