po: explain why make update-po will fail on a VPATH build.

Commit c96006919 worked around a still-existing bug (well at least the
bug report is still open) of intltool with automake 1.15 when the build
and source directories are different (VPATH build) by replacing
`intltool-update` by `true`. But because of this `make update-po` fails
on such a build and the make output is totally unclear at why. Add an
error message when this happens.

po/Makefile.in.in

@@ -97,6 +97,11 @@ all-yes: $(CATALOGS)
 all-no:
 
 $(GETTEXT_PACKAGE).pot: $(POTFILES)
+	@if test "x$(INTLTOOL_UPDATE)" = "x/bin/true"; then \
+		echo "Error: po generation deactivated on VPATH builds."; \
+		echo "       See commit c96006919."; \
+		false; \
+	fi
 	$(GENPOT)
 
 install: install-data