Package: appstream When cross building gthumb, appstreamcli fails to run with an Exec format error. This is a clue that maybe appstream should be marked Multi-Arch: foreign. Such a marking is only correct if appstream provides an architcture-independent interface. It seems that the main way to interact with the appstream package is invoking the appstreamcli tool. This is a good start for Multi-Arch: foreign. What is not clear from the manual page though is whether the architecture of the appstreamcli binary plays a role in its operation or whether it behaves the same way on all architectures. Would you be able to shed some light on this?
If it cannot, gthumb should likely annotate its appstream dependency with :native. Helmut
