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 also comes with its own lightweight and extremely fast Objective-C
runtime, which in real world use cases was found to be significantly faster
than both GNU's and Apple's runtime.

https://objfw.nil.im