This commit to mingw-w64 broke compatibility with emacs-head because of the OS_NT conflict:

    In file included from w32image.c:32:
    w32common.h:45:3: error: expected identifier before numeric constant
       45 |   OS_NT
          |   ^~~~~

https://sourceforge.net/p/mingw-w64/mingw-w64/ci/dc7ef298837d4581200a3bed660aed36818c1b00/

The series of patches in attachments should fix the issue:
- the first one should cause the build to fail in the future instead of silent overriding definition;
- the second patch renames OS_9X to conform mingw-w64 naming;
- the third actually fixes build failure (preserving backward compatibility);

Best regards,
Konstantin Shabanov