https://bugs.gentoo.org/899566
https://github.com/rakshasa/libtorrent/pull/237

From e851a8657d2ef5c332260f1e3a3896f438dba255 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Tue, 3 Jan 2023 14:41:11 +0100
Subject: [PATCH] configure: Define _GNU_SOURCE when checking for
 pthread_setname_np

Previously, the test would always fail with compilers that do not
support implicit function declarations because the pthread_setname_np
function was not declared.
--- a/scripts/checks.m4
+++ b/scripts/checks.m4
@@ -469,6 +469,7 @@ AC_DEFUN([TORRENT_CHECK_PTHREAD_SETNAME_NP], [
   AC_MSG_CHECKING(for pthread_setname_np type)
 
   AC_TRY_LINK([
+    #define _GNU_SOURCE
     #include <pthread.h>
     #include <sys/types.h>
   ],[