This is because upstream builds with -Werror -Wall by default, and that's usually a bad idea for distro packages. I fixed this like this:
.PHONY: override_dh_auto_build
override_dh_auto_build: $(MAINT_SCRIPTS)
- dh_auto_build
+ dh_auto_build -- WERROR=
Not really sure if this is appropriate for Debian or not!