po: Don't create backup .po files

As well as preventing creation of useless backup files, it works
around a bug in gettext 0.17 on OS/X

po/Makevars

@@ -42,6 +42,6 @@ EXTRA_LOCALE_CATEGORIES =
 
 # Avoid line numbers in *.po, but keep them in *.pot.
 MSGMERGE = msgmerge --no-location
-MSGMERGE_UPDATE = msgmerge --no-location --update
+MSGMERGE_UPDATE = msgmerge --no-location --update --backup=off
 MSGFILTER = msgfilter --no-location