mac frameworks: link target alias to Current version

by convention, $target.framework/$target links to
Versions/Current/$target, not Versions/$version/$target.
Versions/Current already links to $version.
so this adds one indirection, but is otherwise the same.

Change-Id: If3d1a3713712f4221ec31883977e50bce6f91764
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>