Stockfish DD

Stockfish bench signature is: 8596156
Restore development version

bench: 8596156
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.

No functional change.
Merge branch 'master' into adaptive_contempt