There are currently 3 different mechanisms being used for frameworks,
which is of course intolerable so we invent a 4th one and standardize on
it:  gb_LinkTarget_use_darwin_frameworks

(This doesn't mean using add_libs or externals was wrong, it was just
inconsistent... and i don't see an obvious benefit of using externals here)

Change-Id: I5de9020402c87e7236c6a358c47f02fa56642d3d