Use absolute filename to `babel.cfg`.

src/i18n/Makefile.am

@@ -33,7 +33,7 @@ build: update
 
 extract:
 	@if [ "$(PYBABEL)" = "" ]; then echo "Command 'pybabel' not found, required when extracting i18n"; exit 1; fi
-	$(PYBABEL) extract -F babel.cfg --no-default-keywords -k _n:1,2 -k _ \
+	$(PYBABEL) extract -F "$(CURDIR)/babel.cfg" --no-default-keywords -k _n:1,2 -k _ \
 		--no-location --project="$(I18N_PROJECT)" --version="$(I18N_VERSION)" \
 		--copyright-holder="$(I18N_COPYRIGHT)" --msgid-bugs-address="$(I18N_BUG_ADDRESS)" \
 		-o "$(CURDIR)/messages.pot" \