GNU Objective-C Runtime still does not support objc_getAssociatedObject and friends

[wyatt8740]

From objc/runtime.h:

/* Compatibility Note: The Apple/NeXT runtime has the functions
   objc_setAssociatedObject (), objc_getAssociatedObject (),
   objc_removeAssociatedObjects () and the objc_AssociationPolicy type
   and related enum.  The GNU runtime does not have them yet.
   TODO: Implement them.  */

Thus, this project still does not build under GCC's objective-C compiler (this is related to issue #2, by the way).

I would really prefer not to have to install all of LLVM + Clang just to build a theme.

I should add for anyone else who sees this that if I do install clang and compile GNUstep's libobjc2 with it (https://github.com/gnustep/libobjc2/), it builds. Just remember that you need to use clang both to build libobjc2 and to build Rik.