Make Clang the default makespec on Mac OS 10.7 (Lion) if Clang >= 3.0

Make Clang the default makespec on Mac OS 10.7 (Lion) if Clang >= 3.0

Change-Id: Ibeb1db0eca2fb6d843d85c3f48c5f23a81230745
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>
Reviewed-by: Zeno Albisser <zeno.albisser@nokia.com>
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>
Reviewed-by: Sergio Ahumada <sergio.ahumada@nokia.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>