[libobjc] May I tweak objc-api.h?

[Nicola Pero]

> > But the structs are used in GNUStep and which cannot change just 
> > support one version of
> > gcc.  They have to support multiple versions of gcc.  I rescind my ok 
> > on this.  Knowing that
> > programs will brake.  Also changing it conditional on __cplusplus is 
> > okay with me as it is
> > okay with Alex.
> 
> Hey, we're all supposed to be OO thinkers: How about we introduce an 
> accessor macro for this field, called say OBJC_SUPER_CLASS.

That wouldn't help much since previous release of libobjc don't define it.  
You'd still have to make a difference between new libojcs (where you can
use the macro) and old ones (where you can't use the macro).

At that point, it's entirely equivalent but simpler to use .class for old
libobjcs and .super_class for new ones.