Add -fvisibility=hidden flag to OS X apps/ builds
Add -fvisibility=hidden flag to OS X apps/ builds

This removes a large number of harmless-but-annoying warnings when
building test_apps for OS X

ronen/Halide,kgnk/Halide,ronen/Halide,kgnk/Halide,psuriana/Halide,psuriana/Halide,tdenniston/Halide,kgnk/Halide,ronen/Halide,tdenniston/Halide,kgnk/Halide,psuriana/Halide,jiawen/Halide,ronen/Halide,psuriana/Halide,jiawen/Halide,tdenniston/Halide,psuriana/Halide,ronen/Halide,kgnk/Halide,psuriana/Halide,tdenniston/Halide,kgnk/Halide,ronen/Halide,jiawen/Halide,tdenniston/Halide,jiawen/Halide,jiawen/Halide,ronen/Halide,tdenniston/Halide,jiawen/Halide,jiawen/Halide,kgnk/Halide,ronen/Halide,psuriana/Halide,kgnk/Halide,tdenniston/Halide,tdenniston/Halide