OSX/scons: simplification for getting the system version ( major, minor ), use sw_vers -productVersion instead of uname -r, we must not redine it then from darwin version -> osx version

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61163 954f8c5b-7b00-dc11-b283-0030488c597c