Allow exit code 1 from grep in make linkcheckbroken

Fixes plone#1212

Author: wolbernd

Makefile

@@ -189,7 +189,7 @@ linkcheck: deps  ## Run linkcheck
 
 .PHONY: linkcheckbroken
 linkcheckbroken: deps  ## Run linkcheck and show only broken links
-	cd $(DOCS_DIR) && $(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck | GREP_COLORS='0;31' grep -wi "broken\|redirect" --color=auto
+	cd $(DOCS_DIR) && $(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck | GREP_COLORS='0;31' grep -wi "broken\|redirect" --color=auto || test $$? = 1
 	@echo
 	@echo "Link check complete; look for any errors in the above output " \
 		"or in $(BUILDDIR)/linkcheck/ ."