debian: rules: Work around a regression in Autoconf 2.70.

* packaging/rules/debian/rules (override_dh_auto_build): Invoke the
contrib build system with GTKDOCIZE set to false to workaround
https://savannah.gnu.org/support/?110503 present in Debian unstable.

Change-Id: I03e6d070d33fb126e3777517b6b4f1b4f042e315

packaging/rules/debian/rules

@@ -75,7 +75,7 @@ override_dh_auto_configure:
 	  --disable-natpmp  \
 	  --enable-gnutls $(BUNDLED_PKGS) && \
 	make list && \
-	make -j$(NO_CPUS) V=1
+	GTKDOCIZE=true make -j$(NO_CPUS) V=1
 	cd daemon && \
 	./autogen.sh && \
 	./configure \