It is currently not possible to build Squeekboard with only
packages that are available in Debian.
This is not likely to change until Squeekboard has been ported
to GTK4 or gtk4-layer-shell.
Until then, vendor the dependencies during the build,
so that one can still use the ".deb"-packages for testing.
Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/659>