I've just had a go at building using mingw/msys and was pleasantly surprised that it worked pretty much out of the box. Here are the instructions in case anyone else is interested.
Anyone got any ideas about the couple of issues towards the bottom?
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Download MSYS2 (x86_64 version) from
https://msys2.github.io/Install to c:\msys
run c:\msys\msys2_shell and update msys with:
pacman -Syu
re-run msys2_shell (pacman -Syu should now say all up to date) get packages
pacman -S mingw-w64-i686-toolchain mingw-w64-i686-gtk2 mingw-w64-i686-gtkglext
pacman -S automake autoconf libtool pkg-config flex bison
Extract source from snapshot on website: to c:\msys\home\<username>
run c:\msys\mingw32.exe
change to source dir (cd gnubg - whatever you called it above):
./autogen.sh
./configure
Issue 1: mingw32-make.exe doesn't work as $(MAKE) resolves to make inside the makefiles
Hack: copy mingw32-make to make
cp /mingw32/bin/mingw32-make.exe /mingw32/bin/make.exe
make
make install
Issue 2: running gnubg now gives errors about not finding bear off database etc.
Hack: copy gnubg.exe to /share/gnubg folder and run from there...
cp /mingw32/bin/gnubg.exe /mingw32/share/gnubg/
/mingw32/share/gnubg/gnubg