"server:fixed" was a good default when GTK1 was common and non-X11
environments were rare. Now it's the other way round - Wayland is very
common and the GTK1 configuration of PuTTY is legacy - so it's time to
make the default GTK font a client-side one.
Of course, anyone with an existing saved session (including Default
Settings) won't be affected by this change; it only helps new users
without an existing ~/.putty at all. That's why we _also_ need the
fallbacks introduced by the previous couple of commits. But we can at
least start making it sensible for new users.
(I considered keeping the #if, and switching it round so that it tests
GTK_CHECK_VERSION(2,0,0) rather than NOT_X_WINDOWS, i.e. selects the
client-side default whenever client-side fonts _are_ available,
instead of only when server-side fonts _aren't_. That way, in GTK1
builds, the Conf default font would _still_ be "server:fixed". But I
think this is firstly too marginal to worry about, and secondly, it's
more futureproof to make the default the same everywhere: if anyone
still stuck on a GTK1 environment later manages to update it, then
their saved settings are less likely to have had a legacy thing
written into them. And the GTK1 build will still run out of the box
because of the last-ditch fallback mechanism I've just added.)
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