gcc: use special native system headers for darwin

Darwin systems need to be able to find CoreFoundation headers as well as
libc headers. Somehow, gcc doesn't accept any "framework" parameters
that would normally be used to include CoreFoundation in this
situation.

HACK: Instead, this adds a derivation that combines the two. The result
works but probably not a good long term solution.

ALTERNATIVES: Maybe sending patches in to GCC to allow
"native-system-framework" configure flag to get this found.