Commit 253e11695ba added support for emulating weak-linking against dylibs
that are (under the emulation) absent at runtime. This commit extends emulated
weak linking support to allow a real dylib to supply the interface (i.e.
-weak-lx / -weak_library can be pointed at a dylib, in which case they should
be read as "weak-link against this dylib, behavining as if it weren't actually
present at runtime").
