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/configure.ac b/src/configure.ac
index 2943ec5..b7b4747 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -4520,6 +4520,12 @@
 		[#include <libintl.h>],
 		[dgettext("Test", "Test");])],
 		AC_MSG_RESULT([yes]); AC_DEFINE(HAVE_DGETTEXT),
+      AC_MSG_CHECKING([for dngettext])
+		AC_MSG_RESULT([no]))
+      AC_LINK_IFELSE([AC_LANG_PROGRAM(
+		[#include <libintl.h>],
+		[dngettext("DOMAIN", "Test single", "Test plural", 1);])],
+		AC_MSG_RESULT([yes]); AC_DEFINE(HAVE_DNGETTEXT),
 		AC_MSG_RESULT([no]))
       dnl _nl_msg_cat_cntr is required for GNU gettext
       AC_MSG_CHECKING([for _nl_msg_cat_cntr])