po: add Makevars magic so we don't get line numbers in *.po files

This avoids the number one reason for local modifications in *.po
files and and makes things less annoying when working with git (or
any other VCS for that matter).

po/Makevars

@@ -39,3 +39,8 @@ MSGID_BUGS_ADDRESS = http://bugzilla.gnome.org/
 # This is the list of locale categories, beyond LC_MESSAGES, for which the
 # message catalogs shall be used.  It is usually empty.
 EXTRA_LOCALE_CATEGORIES =
+
+# Avoid line numbers in *.po, but keep them in *.pot.
+MSGMERGE = msgmerge --no-location
+MSGMERGE_UPDATE = msgmerge --no-location --update
+MSGFILTER = msgfilter --no-location