From Frederic Morin, "Current behaviour uses SDKSettings.plist to determine osx version but
this file is only available when XCode is installed.

This version works also when Command Line Tools for XCode are
installed (minimum build env for osx without XCode)

See https://github.com/openscenegraph/osg/pull/8 for patch details
Patch: https://github.com/openscenegraph/osg/pull/8/files"