https://bugs.gentoo.org/830222
https://github.com/cgdb/cgdb/pull/338

From 039dba0ec3ae2c4b2636418652a5c63f9ff86049 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Wed, 8 Mar 2023 11:29:06 +0100
Subject: [PATCH] configure: Avoid implicit int in readline check

Future compilers will not accept implicit ints by default, altering
the outcome of the check without this change.

Signed-off-by: Florian Weimer <fweimer@redhat.com>
--- a/config/readline_check_version.m4
+++ b/config/readline_check_version.m4
@@ -86,7 +86,7 @@ AC_CACHE_VAL(ac_cv_rl_version,
 #include <stdlib.h>
 #include <readline/readline.h>
 
-main()
+int main(void)
 {
 	FILE *fp;
 	fp = fopen("conftest.rlv", "w");