https://github.com/shemminger/iproute2/commit/d5fe96ab70928508f072a47449e9b641e46de323

From d5fe96ab70928508f072a47449e9b641e46de323 Mon Sep 17 00:00:00 2001
From: Khem Raj <raj.khem@gmail.com>
Date: Wed, 10 Aug 2022 22:34:40 -0700
Subject: [PATCH] configure: Define _GNU_SOURCE when checking for setns

glibc defines this function only as gnu extention

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
--- a/configure
+++ b/configure
@@ -191,6 +191,7 @@ check_ipt_lib_dir()
 check_setns()
 {
     cat >$TMPDIR/setnstest.c <<EOF
+#define _GNU_SOURCE
 #include <sched.h>
 int main(int argc, char **argv)
 {