Translations: Use the msgmerge option --add-location=file

This way the PO file diffs are less noisy but the locations of the
strings are still present at file level, just without line numbers.

The option is available since gettext 0.19 (2014).
configure.ac requires 0.19.6.

po/Makevars

@@ -63,7 +63,7 @@ USE_MSGCTXT = no
 # Useful options are in particular:
 #   --previous            to keep previous msgids of translated messages,
 #   --quiet               to reduce the verbosity.
-MSGMERGE_OPTIONS = --no-wrap
+MSGMERGE_OPTIONS = --add-location=file --no-wrap
 
 # These options get passed to msginit.
 # If you want to disable line wrapping when writing PO files, add