Add more options to XGETTEXT_OPTIONS in po/Makevars

--add-comments makes translator comments appear in the .po files, and the
rest is future-proofing (in case we need msgctxt and msgid_plural later).

po/Makevars

@@ -8,7 +8,7 @@ subdir = po
 top_builddir = ..
 
 # These options get passed to xgettext.
-XGETTEXT_OPTIONS = --keyword=_ --keyword=N_
+XGETTEXT_OPTIONS = --from-code=UTF-8 --keyword=_ --keyword=N_ --keyword=C_:1c,2 --keyword=NC_:1c,2 --keyword=g_dngettext:2,3 --add-comments
 
 # This is the copyright holder that gets inserted into the header of the
 # $(DOMAIN).pot file.  Set this to the copyright holder of the surrounding