po: Make sure all sane-backends.pot input are available

po/Makevars

@@ -1,5 +1,6 @@
 #  Makevars -- snippet for inclusion in po/ -*- Makefile -*-
-#  Copyright (C) 2016, 2019  Olaf Meeuwissen and the SANE developers
+#  Copyright (C) 2016, 2019, 2020  Olaf Meeuwissen
+#    and the SANE developers
 #
 #  This file is part of the SANE build infra-structure.  See the
 #  included LICENSE file for license information.
@@ -47,3 +48,8 @@ width_options     = --width=75
 #  Locale categories, in addition to LC_MESSAGES, for which message
 #  catalogs are to be used.
 EXTRA_LOCALE_CATEGORIES =
+
+#  Selected files in POTFILES.in are generated.  Make sure they are
+#  available when needed.
+$(top_srcdir)/backend/pixma/pixma_sane_options.c:
+	cd ../backend; make pixma/pixma_sane_options.c