Check for requirements when executing `make styleshint`.

src/styles/Makefile.am

@@ -38,4 +38,6 @@ styles:
 		$(CURDIR)/font-awesome.scss:$(STATIC)/css/font-awesome.min.css
 
 styleshint:
+	@if [ "$(SASS)" = "" ]; then echo "Command 'sass' not found, required when checking styles"; exit 1; fi
+	@if [ "$(SASS_SUPPORT_STYLES)" = "no" ]; then echo "Your version of sass does not support checking styles"; exit 1; fi
 	$(FIND) ./ -maxdepth 1 -name "*.scss" -print0 | xargs -0 -n1 $(SASS) --compass --scss $(SASSFLAGS) -c