osx build: make 10.7 default deployment target

10.6 isn't shipped with xcode > 4.4 anymore per default
If older/newer deployment target is required it can be set with
-DCMAKE_OSX_DEPLOYMENT_TARGET=10.x
Merge pull request #1502 from bmiklautz/osx_deployment_target

osx build: make 10.7 default deployment target