Merge: Better Objective-C FFI and cleanup for OS X

This PR opens the way for the iOS platform support.

Two commits may look very similar: "ObjC FFI: import public Objective-C headers for imported modules" and "nitc: fix duplication on imports of public foreign code". They both manage importing headers from other modules so public extern classes may work across module, but one is for Objective-C and the other for C. This service will have to be implemented for other languages too (mostly C++), I'll be looking for a higher level service at that time.

Pull-Request: #1105
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>