patch 9.1.1064: not possible to use plural forms with gettext()

Problem:  not possible to use plural forms with gettext()
Solution: implement ngettext() Vim script function (Christ van Willegen)

closes: #16561

Signed-off-by: Christ van Willegen <cvwillegen@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/config.h.in b/src/config.h.in
index 3ff4605..79cb37c 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -430,6 +430,9 @@
 /* Define if there is a working dgettext(). */
 #undef HAVE_DGETTEXT
 
+/* Define if there is a working dngettext(). */
+#undef HAVE_DNGETTEXT
+
 /* Define if _nl_msg_cat_cntr is present. */
 #undef HAVE_NL_MSG_CAT_CNTR