Windows build configuration

[Kai Willadsen <kai willadsen gmail com>]

Hi all,

I finally got around to cutting the 3.18.1 release, only to discover
that Appveyor won't properly build from tags given the way our release
process works. I've re-enabled per-commit builds now, so artifacts
from the 3.18 branch are available (e.g.,
https://ci.appveyor.com/project/kaiw/meld/build/meld-3-18-e5f6456.18/artifacts).

However, this still doesn't work right. Because it's not building the
tag itself (and because we do post-release version bumping), the
current build that's available is built as though it's 3.18.2, which
it definitely isn't.

I'll see if I can figure out some way of dealing with the incorrect
versioning, but that won't be immediate.

As an unrelated but extremely annoying issue, Appveyor is currently
building off the "meld" Github organisation, which is a
manually-maintained mirror. Now that we're hosted on GNOME's GitLab
instance, I feel like we should be able to use GitLab hooks instead,
but... that's more work to figure out.

cheers,
Kai