calf cannot be cross built from source, because its dependency on
libjs-jquery-colorbox cannot be satisfied. In general, Architecture: all
packages can never satisfy cross Build-Depends unless marked Multi-Arch:
foreign or annotated :native. In this case, the foreign marking is
reasonable, because libjs-jquery-colorbox is a pure javascript library
with no maintainer scripts and all of its dependencies are already thus
marked. Please consider applying the attached patch.

Helmut