https://github.com/WayneD/rsync/pull/558

From d869bcd3850b9a80f27f5aee4d41725f39d84ea7 Mon Sep 17 00:00:00 2001
From: Ivan Babrou <github@ivan.computer>
Date: Mon, 1 Jan 2024 19:31:01 -0800
Subject: [PATCH] configure.ac: fix failing IPv6 check due to missing return
 type

Fixing this warning escalated to an error, resuting in no IPv6 support:

```
configure.sh:7679: checking whether to enable ipv6
configure.sh:7718: clang -o conftest -g -O2 -DHAVE_CONFIG_H -Wall -W   conftest.c  >&5
conftest.c:73:1: error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]
main()
^
int
1 error generated.
configure.sh:7718: $? = 1
configure.sh: program exited with status 1
```
--- a/configure.ac
+++ b/configure.ac
@@ -392,7 +392,7 @@ AS_HELP_STRING([--disable-ipv6],[disable to omit ipv6 support]),
 #include <stdlib.h>
 #include <sys/types.h>
 #include <sys/socket.h>
-main()
+int main()
 {
    if (socket(AF_INET6, SOCK_STREAM, 0) < 0)
      exit(1);