VS 2026 support on old branches

Greetings,

Peter E pinged me recently about the need to upgrade drongo because he was
planning to commit some stuff that would break on VS 2019. I chose to
update it to VS2026, and this promptly broke on the old branches. I've
hacked its config for now to use the older setup for 14 and 15, but that's
not very principled - and buildfarm members are supposed to use the same
compiler for all branches. If anyone wants to set up a buildfarm animal now
they will have to choose between using an older compiler (VS2022) and not
supporting all the live branches. So here's a patch (which I have tested)
that adds support for VS2026 on those old branches, pretty much in line
with what we've done before. With any luck, Microsoft won't come out with
another release until after the date our release 15 goes EOL, at which
point we will be done with this kabuki dance.

cheers

andrew