The MSYS2 ecosystem is available at https://www.msys2.org/ and builds upon earlier work by MinGW-w64 (in turn a fork of MinGW.org (aka mingw-w32)) and Cygwin projects, to name a few related efforts. It also includes pacman similar to that in Arch Linux for easier dependency installation, and many packages are available "out of the box" this way.

The project is currently sponsored by Microsoft and seems to be supported by Visual Studio Code IDE for building and debugging projects, for more details see https://code.visualstudio.com/docs/cpp/config-mingw

Notable pages of the project include:

After downloading and installing MSYS2 archive for the first time, they suggest to start by updating the base ecosystem (using their terminal):

:; pacman -Syu

Wait for metadata and base package downloads, agree that all MSYS2 programs including your terminal would be closed/restarted, and wait for this stage to complete.

Run it again to refresh more of the ecosystem, now without restarting it:

:; pacman -Syu

Finally, install tools and prerequisites for building NUT; note that some of the recommended package names are "umbrellas" for several implementations, and the pacman would ask you which (or "all") to install in those cases.

To avoid toolkit variant mismatches, you may require to use their specific builds preferentially:

PATH="/mingw64/bin:$PATH"
export PATH

…and also add these lines to the ~/.bashrc file.

# This covers most of the common FOSS development baseline, including
# python, perl, autotools, gcc, clang, git, binutils, make, pkgconf...
:; pacman -S --needed \
    base-devel mingw-w64-x86_64-toolchain \
    autoconf-wrapper automake-wrapper libtool mingw-w64-x86_64-libltdl \
    clang gcc \
    ccache mingw-w64-x86_64-ccache \
    git aspell aspell-en \
    vim python \
    mingw-w64-x86_64-python-pygments

# PThreads come as an extra feature; note there are many variants,
# see https://packages.msys2.org/search?t=binpkg&q=pthread
:; pacman -S --needed \
    mingw-w64-x86_64-winpthreads-git \
    mingw-w64-clang-x86_64-winpthreads-git

# Note that MSYS2 includes libusb-1.0 "natively"
# The NUT codebase adjustments for Windows might at this moment expect older
# ecosystem via https://github.com/mcuee/libusb-win32 -- subject to fix then.
:; pacman -S --needed \
    mingw-w64-x86_64-libusb \
    mingw-w64-clang-x86_64-libusb

# Seems that the older libusb-win32 (libusb-0.1) is also available as:
:; pacman -S --needed \
    mingw-w64-x86_64-libusb-win32 \
    mingw-w64-clang-x86_64-libusb-win32

# Alternately there is libusb-compat (libusb-1.0 codebase exposing the older
# libusb-0.1 API) which SHOULD NOT be installed along with the real libusb-0.1:
# :; pacman -S --needed mingw-w64-x86_64-libusb-compat-git mingw-w64-clang-x86_64-libusb-compat-git

# This also pulls *-devel of several other projects:
:; pacman -S --needed \
    mingw-w64-x86_64-neon libneon-devel

# Other dependencies:
:; pacman -S --needed \
    mingw-w64-x86_64-libmodbus-git \
    mingw-w64-clang-x86_64-libmodbus-git \
    mingw-w64-x86_64-libgd \
    mingw-w64-clang-x86_64-libgd

# For C++ tests:
:; pacman -S --needed \
    mingw-w64-x86_64-cppunit \
    mingw-w64-clang-x86_64-cppunit

ccache wrapper scripts are available as e.g. /mingw64/lib/ccache/bin/gcc and lack a set for clang tools; easy-peasy fix with:

:; cd /mingw64/lib/ccache/bin
:; for T in clang clang++ clang-cpp ; do sed "s/gcc/$T/" < gcc > "$T" ; chmod +x "$T" ; done

Note that default ccache seems quirky on Windows MSYS2, possibly due to mixing of the path separator characters and/or embedding and choking on the C: in path names. Overall it seems unable to create the cache files after it has created the cache directory tree (though you might have to pre-create the ${HOME}/.ccache anyway, as NUT ci_build.sh script does. As found in experimentation, setting the PATH consistently for toolkits involved is very important.

Notable packages not found in the repo:

Not installed above (yet?):

Note that ccache symlinks for MSYS2 are installed into /usr/lib/ccache/bin directory (not plain /usr/lib/ccache as elsewhere).

:; find "$DESTDIR" -name '*.exe' -type f  | while read F ; do ldd "$F" \
   | grep ' /mingw64/' ; done  | awk '{print $3}' | sort | uniq \
   | while read LIB ; do cp -pf "$LIB" "$DESTDIR/mingw64/bin/" ; done

Keep in mind that a similar trick (or links to *.dll — and symlinks are problematic on that platform) may be needed in other directories, such as sbin and cgi-bin:

:; ( cd "$DESTDIR/mingw64/bin/" && ln *.dll ../sbin && ln *.dll ../cgi-bin )