Update po Makefile to regenerate .po file on .pot update

Weblate updates its translations only by checking the .po files, not the
.pot file.

Author: jocelynj

web/po/Makefile

@@ -7,7 +7,7 @@ MO:=$(patsubst %.po,mo/%/LC_MESSAGES/osmose-frontend.mo,$(PO))
 
 .PRECIOUS: $(PO) $(MO)
 
-all: osmose-frontend.pot mo
+all: osmose-frontend.pot $(PO) mo
 
 ../node_modules/easygettext/src/extract-cli.js:
 	$(error 'easygettext missing. Please install node dependencies with `npm install`')
@@ -34,6 +34,9 @@ osmose-frontend.pot: python.pot vue.pot
 	mv $@.tmp $@
 	rm $@.nofiles
 
+%.po: osmose-frontend.pot
+	msgmerge --previous --update $@ osmose-frontend.pot
+
 mo: $(MO)
 
 mo/%/LC_MESSAGES/osmose-frontend.mo: %.po