https://lore.kernel.org/netdev/20231202024705.1375296-1-sam@gentoo.org/T/#u

From 3dc0bf34c1571429c08a8d0c08a69502e3e4575c Mon Sep 17 00:00:00 2001
From: Sam James <sam@gentoo.org>
Date: Sat, 2 Dec 2023 02:46:05 +0000
Subject: [PATCH] configure: Add _GNU_SOURCE to strlcpy configure test

>=glibc-2.38 adds strlcpy but it's guarded under a feature-test macro. Just
add _GNU_SOURCE to the configure test because we already pass _GNU_SOURCE unconditionally
in the Makefiles when building iproute2.

Signed-off-by: Sam James <sam@gentoo.org>
--- a/configure
+++ b/configure
@@ -445,6 +445,7 @@ EOF
 check_strlcpy()
 {
     cat >$TMPDIR/strtest.c <<EOF
+#define _GNU_SOURCE
 #include <string.h>
 int main(int argc, char **argv) {
 	char dst[10];
-- 
2.43.0