https://bugs.gentoo.org/899812
https://github.com/dajobe/flickcurl/pull/43

From 940019e2635f1bc21b712fadbf4f73ca45a26379 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Fri, 13 Jan 2023 09:33:08 +0100
Subject: [PATCH] configure: Include <stdio.h> in vsnprintf check

Avoid implicit declaration of vsnprintf for improved C99
compatibility.  Otherwise the configure script will fail
to detect vsnprintf support with future compilers which
do not support implicit function declarations.
--- a/configure.ac
+++ b/configure.ac
@@ -133,6 +133,7 @@ AC_LINK_IFELSE([AC_LANG_PROGRAM([[#ifdef HAVE_GETOPT_H
 if test $ac_cv_func_vsnprintf = yes; then
   AC_MSG_CHECKING(vsnprintf has C99 compatible return value)
   AC_RUN_IFELSE([AC_LANG_SOURCE([[#include <stdarg.h>
+#include <stdio.h>
 int is_c99(char *s, ...) {
   char buffer[32];
   va_list args;