Update Makefile for Mac OS X compilation

Update Makefile for Mac OS X compilation

This change in the Makefile restores the possibility to compile
Stockfish on Mac OS X 10.9 and 10.10 after the C++11 has been merged.

To use the default (fastest) settings, compile with:

make build ARCH=x86-64-modern

To test the clang settings, compile with

make build ARCH=x86-64-modern COMP=clang

Beware that the clang settings may provide a slightly slower (6%)
executable.

Backported from master.

No functional change

Resolves #275