ObjFW is a portable, lightweight framework for the Objective-C language. It enables you to write an application in Objective-C that will run on any platform supported by ObjFW without having to worry about differences between operating systems or various frameworks you would otherwise need if you want to be portable.

It supports all modern Objective-C features when using Clang, but is also compatible with GCC ≥ 4.6 to allow maximum portability.

ObjFW is intentionally incompatible with Foundation.

You can read more about ObjFW on its homepage, including how to install and use it.