https://bugs.gentoo.org/874687
from
https://github.com/jedsoft/slrn/pull/1
edited to current situation (line numbers changed so patch applies cleanly)
and half of the patch pertaining autotools is dropped

From f550d0ab9e1cd3cfe844022435c3d98af3c18441 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Sat, 4 Feb 2023 15:04:36 +0100
Subject: [PATCH] autoconf: Fix C99 compatibility issue in the VA_COPY checks

The exit function is called without including <stdlib.h>, resulting
in an implicit function declarations.  Future compilers will not
support implicit function declarations by default.  This will lead
to build failures.
---
 autoconf/configure.ac | 3 +++
 configure             | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/configure b/configure
index faec350..8758172 100755
--- a/configure
+++ b/configure
@@ -8307,6 +8307,7 @@ else
 /* end confdefs.h.  */
 
 	#include <stdarg.h>
+	#include <stdlib.h>
 	void f (int i, ...) {
 	va_list args1, args2;
 	va_start (args1, i);
@@ -8352,6 +8353,7 @@ else
 /* end confdefs.h.  */
 
 	#include <stdarg.h>
+	#include <stdlib.h>
 	void f (int i, ...) {
 	va_list args1, args2;
 	va_start (args1, i);
@@ -8397,6 +8399,7 @@ else
 /* end confdefs.h.  */
 
 	#include <stdarg.h>
+	#include <stdlib.h>
 	void f (int i, ...) {
 	va_list args1, args2;
 	va_start (args1, i);