libobjc: should nil be an id?

[Daniel Ferreira (theiostream)]

Hi,

In libobjc2, `nil` is defined as a null pointer cast to an `id`[1].
However, this breaks a lot of stuff targeted to Apple's libobjc which
relies on `nil` and `NULL` being interchangeable.

I've experimented removing that cast from the runtime.h header and
this works fine for compiling WebKit. This breaks libs-base and
libs-gui, though, with stuff like:

> NSKeyedUnarchiver.m:862:33: error: cast to union type from type 'void *' not 
> present in union
>                        GSIArrayAddItem(_objMap, (GSIArrayItem)nil);

How do you suggest handling this issue?

-- Daniel.

[1]: https://github.com/gnustep/libobjc2/blob/master/objc/runtime.h#L207