Linking with -weak_framework is recently possible on nightly:

#![feature(link_arg_attribute)]

#[link(name = "-weak_framework", kind = "link-arg", modifiers = "+verbatim")]
#[link(name = "CoreFoundation", kind = "link-arg", modifiers = "+verbatim")]
extern "C" {}

This works even if declared in a binary crate that depends on something that has the normal #[link(name = "CoreFoundation", kind = "framework")].

Given that, maybe we should consider removing the link feature again?