I believe that this is a bug in clang. If the class is dllexported then the ivar offset variables for public / protected ivars should be dllexported as well. I think I missed that case when doing the Objective-C support for Windows - I'll take a look tomorrow.
Thanks. I tried explicitly marking these variables as @public in Base but are getting the same error.
By the way, I've been using vcpkg for another project (on FreeBSD) - it's probably the best way of getting the dependencies for any libraries that GNUstep depends on for the MSVC ABI. It has good CMake integration but can probably be made to work with GNUstep Make (though if you have a CMake build system for GNUstep, it should work very nicely).
Oh vcpkg looks nice indeed, thanks for the pointer! I already got all the dependencies built using custom scripts, but this would certainly make things easier.