I updated the repository with the offending code compiled only if _WIN32_WINNT >= 0x0600, which is not the case for mingw as far as I can see. This will make @check_unicode more optimistic, replying that every character is available, similar to what is suggested in The Z-Machine and @check_unicode.
If you are only looking for binaries, there are automatic appveyor builds for every commit to my tree, the latest is here.