See scripts/Windows/README.adoc for original recommendations for the effort,
including possibilities of cross-builds with mingw available in Linux.
Unfortunately these did not work for me at the time of testing, yielding some issues downloading mingw both in Windows and Linux environments. So I explored other downloads, as detailed below.
See also:
Seems the mingw installer has problems with current authentication
and redirect on SourceForge. You can download and unpack 7z archives from
https://sourceforge.net/projects/mingw-w64/files/mingw-w64/mingw-w64-release/
into e.g. C:\Progra~1\mingw-w64\x86_64-8.1.0-release-posix-seh-rt_v6-rev0
location on your Windows system. Then for building further NUT dependencies
see scripts/Windows/README.adoc.