https://github.com/unrealircd/unrealircd/pull/242

From 3246e472842237ac9e655e254380c4dd8fea7abc Mon Sep 17 00:00:00 2001
From: Sam James <sam@gentoo.org>
Date: Sun, 18 Dec 2022 06:18:51 +0000
Subject: [PATCH] configure.ac: fix -Wimplicit-function-declaration

Clang 16 makes -Wimplicit-function-declaration error by default.

Unfortunately, this can lead to misconfiguration or miscompilation of software as configure
tests may then return the wrong result.

We also fix -Wstrict-prototypes while here as it's easy to do and it prepares
us for C23.

Fixes the following:
```
configure:6250: checking if your system has IPv6 support
configure:6275: x86_64-pc-linux-gnu-gcc -o conftest -O2 -pipe -march=native -fdiagnostics-color=always -frecord-gcc-switches -Wreturn-type      -ggdb3 -Werror=implicit-function-declaration -Werror=implicit-int -fno-strict-aliasing -fno-common -funsigned-char -Wall -Wextra -Waggregate-return -Wformat-nonliteral -Wduplicated-cond -Wduplicated-branches -Wparentheses -Wno-pointer-sign -Wno-format-zero-length -Wno-format-truncation -Wno-format-overflow -Wno-unused -Wno-unused-parameter -Wno-unused-but-set-parameter -Wno-char-subscripts -Wno-sign-compare -Wno-empty-body -Wno-pragmas  -Wl,-O1 -Wl,--as-needed -Wl,--defsym=__gentoo_check_ldflags__=0 -Wl,-z,pack-relative-relocs conftest.c  >&5
conftest.c: In function 'main':
conftest.c:30:1: error: implicit declaration of function 'exit' [-Werror=implicit-function-declaration]
   30 | exit(0); /* We only check if the code compiles, that's enough. We can deal with missing runtime IPv6 */
      | ^~~~
conftest.c:28:1: note: include '<stdlib.h>' or provide a declaration of 'exit'
   27 | #include <sys/socket.h>
  +++ |+#include <stdlib.h>
   28 | int main() {
conftest.c:30:1: warning: incompatible implicit declaration of built-in function 'exit' [-Wbuiltin-declaration-mismatch]
   30 | exit(0); /* We only check if the code compiles, that's enough. We can deal with missing runtime IPv6 */
      | ^~~~
conftest.c:30:1: note: include '<stdlib.h>' or provide a declaration of 'exit'
cc1: some warnings being treated as errors
configure:6275: $? = 1
```

For more information, see LWN.net [0] or LLVM's Discourse [1], the Gentoo wiki [2],
or the (new) c-std-porting mailing list [3].

[0] https://lwn.net/Articles/913505/
[1] https://discourse.llvm.org/t/configure-script-breakage-with-the-new-werror-implicit-function-declaration/65213
[2] https://wiki.gentoo.org/wiki/Modern_C_porting
[3] hosted at lists.linux.dev.
--- a/configure.ac
+++ b/configure.ac
@@ -326,7 +326,7 @@ fi
 ])
 AC_CACHE_CHECK(if your system prepends an underscore on symbols,ac_cv_underscore,[
 cat >uscore.c << __EOF__
-int main() {
+int main(void) {
 	return 0;
 }
 __EOF__
@@ -354,7 +354,8 @@ AC_CACHE_CHECK([if your system has IPv6 support], [ac_cv_ip6], [
 AC_TRY_RUN([
 #include <sys/types.h>
 #include <sys/socket.h>
-int main() {
+#include <stdlib.h>
+int main(void) {
 int s = socket(AF_INET6, SOCK_STREAM, 0);
 exit(0); /* We only check if the code compiles, that's enough. We can deal with missing runtime IPv6 */
 }