I've eventually discovered that MSYS2 pacman can deliver Win32 binaries of the very packages I was struggling to build.
(This is probably something that many others would have known ... but not those of us who are still living in the 1990's ;-)

So I'll settle for that for now.
Thanks for the feedback, and sorry for the noise. (I actually did learn quite a lot from this.)

Cheers,
Rob