mate-desktop fails to cross build from source, because it fails running
the gtk-doc scanner with an Exec format error. This is fairly common.
Fortunately, it also separates its documentation into an Arch:all
package. Hence, we can disable gtk-doc in arch-only builds. With a bit
of tweaking of debian/rules, this makes a cross build succeed. I'm
attaching a patch for your convenience.

Helmut