Fail builds if links are invalid

Author: gerhardol

Makefile

@@ -41,7 +41,7 @@ clean:
 	-rm -rf $(BUILDDIR)/*
 
 html:
-	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	$(SPHINXBUILD) -W -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."