I believe this was due to a temporary mismatch in Debian, between valac and gtk3.
Valac 0.28 generates some code that requires gtk 3.16, which only went into unstable on 6/14. (that build log shows undefined symbols that were only introduced in gtk 3.16, not a valac error) I (the maintainer of deja-dup) can compile just fine with both valac 0.28 and gtk 3.16. Try to build deja-dup again? -mt
