I know, I know. Qt on Linux can now abstract over Wayland and X11. That's
why we don't have Q_WS_X11 anymore. Replacing it by Q_OS_LINUX is worse,
thgough.

Besides, real- world applications still need to know about the windowsystem
because Qt doesn't cover everything. This replaces Q_WS_X11 with HAVE_X11
(cribbed from Digikam).

Unfortunately, Krita is now even more broken than it was, but it's still a
step forward, hard as that might be to believe.