Check for zero-sized Graphvix config file on Ubuntu; VPP-396

- The previous change only accounted for a missing Graphviz config
  file; apparently it can be zero-sized too.

Change-Id: Ic6957d10cdc7cb7b9da72d2b2a0f8913100870c5
Signed-off-by: Chris Luke <chrisy@flirble.org>

doxygen/Makefile

@@ -113,7 +113,8 @@ ifeq ($(OS_ID),ubuntu)
 			dpkg-query --show $$i >/dev/null 2>&1 || inst="$$inst $$i"; \
 		done; \
 		if [ "$$inst" ]; then sudo apt-get $(CONFIRM) $(FORCE) install $$inst; fi
-	@if [ ! -f /usr/lib/graphviz/config6a ]; then \
+	@if [ ! -s /usr/lib/graphviz/config6a ]; then \
+		echo "Rebuidlding system Graphviz configuration."; \
 		sudo dot -c; \
 	fi
 else ifneq ("$(wildcard /etc/redhat-release)","")