Thank you for your answer.
I tried without the unsafe flag, and now on Windows I indeed get a lot of linker warnings "locally defined symbol imported" (for parts produced from Swift sources!), compare this other topic. The resulting program seems to work fine, but yes, it would be nice if this could be fixed (someone help?).
One more little "barrier" for the use of your fork is that your changes reside inside a "development" branch (branches are also judged as "unstable" by the SPM), I needed to fork your repo, merge this branch into main and also needed to set a version tag. (I will not link here to my fork since I think your fork should be the deciding one, but this might be reason that so many forks exist of your repo. Or what about a pull request for weichsel/ZIPFoundation at some point?)