FWIW, it's not really useful for building Wine itself. I think that it's used only in "export all" case on PE targets and we don't use that. We could probably still expose that in headers for winelib. For that, something like "|| __has_attribute(visibility)" should be a bit more flexible.
Given that it doesn't exist on Windows, I don't think it needs to be available for Winelib.
In fact, I'd say that if we no longer need DECLSPEC_HIDDEN in Wine itself, it should eventually be removed.