Avoid warning messages when generating the gallery.

Makefile.in

@@ -708,7 +708,7 @@ gallery-subs::
 	  echo "exporting examples/$$d"; \
 	  mkdir -p $(GALLERYDIR)/$$d; \
 	  cd examples/$$d; \
-	  WHY3CONFIG="" ../../bin/why3doc.@OCAMLBEST@ -L ../../stdlib -L . --stdlib-url http://why3.lri.fr/stdlib/ *.mlw -o $(GALLERYDIR)/$$d; \
+	  WHY3CONFIG="" ../../bin/why3doc.@OCAMLBEST@ -L ../../stdlib -L . --stdlib-url http://why3.lri.fr/stdlib/ --debug ignore_unused_vars *.mlw -o $(GALLERYDIR)/$$d; \
 	  cd ..; \
 	  rm -f $(GALLERYDIR)/$$d/$$d.zip; \
 	  git archive --format=zip -o $(GALLERYDIR)/$$d/$$d.zip HEAD $$d; \