Always link in gb_STDLIBS, except when the library explicitly opts out
with gb_LinkTarget_disable_standard_system_libs.
Change-Id: I489a99114fbfa46d0421a27cf6c7b899dc268a4a
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
