Fix crash on export to filename with $(LABELS)
GETTEXT(3): The resulting string is statically allocated and must not be modified or freed.

Was broken by c0a900978328d7812048ce071cc31cd5b94d2f4e
Bad commit was found using git bisect.

Thanks to Stéphane Gourichon for initial bugreport (on IRC)
Merge pull request #723 from MRIG/master

added 3 files into /po/POTFILES.c
Updated Dutch translation