Change xdg-utils fetchgit url to `http` alternative since it is
more proxy-friendly. The list of repository url alternatives
can be found at http://cgit.freedesktop.org/xdg/xdg-utils/
Fixes#6193.
Disabling docs generation might be another alternative
to the build-time dependency blowup.
That KDE patch no longer applies, but the code seems to take KDE5 into
account already. CC @ttuegel.