Merge request submitted upstream. Hopefully it's pretty close to being up to their standards.

If all goes well, there will soon be an MXE package for libbotan2.

I already have another contribution in the pipeline (libargon2), but I'm going to wait for feedback on libboton2 before I submit a MR. That way the reviewer won't have to give me the same feedback twice. I can just submit libargon2 correctly the first time, hopefully. 🤞