Fix missing macro definitions in apple-apple-apple build.

[iamleeg]

Some GNUstep-specific macros like GS_FOR_IN and GS_FOR_END were defined in Foundation headers, like Foundation/NSEnumerator.h. On macOS, these headers don't get found because they shadow the Apple headers with the same name, so the build fails. This PR pulls those macros into files in GNUstepBase and imports the new files from their old locations.

The base additions library didn't build on macOS (26.3.1 with Xcode 26.3) before these changes, and now does.

[fredkiefer]

Looks good to me

[rfm]

Thanks, I've gone a different way because the fast enumeration changes were really a much more extensive work in progress, and I hadn't realised that I'd left the OSX base additions build in a broken state.
The code changes as a whole are supposed to make fast enumeration available on all platforms (using native compiler/runtime support where possible) and to make it possible to write portable code (using the macros) without having to include additional headers.
I hope that's all working now, and in particular that it is working on OSX and windows. At least the OSX code built for me last time I tried (though I didn't actually run tests).

[rfm]

I forgot to say: thanks very much for spotting the issue, and pointing it out. If you hadn't done that I might have completely forgotten it and left it for ages.

[iamleeg]

Great, thank you!