From: https://github.com/knxd/knxd/pull/531/commits/cb8a564d2c6250f712528e929892a5e7ddb74778

From cb8a564d2c6250f712528e929892a5e7ddb74778 Mon Sep 17 00:00:00 2001
From: "Sergey V. Lobanov" <sergey@lobanov.in>
Date: Mon, 31 Jan 2022 01:34:20 +0300
Subject: [PATCH] Add clang suport as build cc

ccforbuild.m4 comes from gmp project (very old version).
Current version does not support clang due to using GCC exit()
builtin function. clang does not have exit() builtin function.

GMP project fixed this issue by the commit https://gmplib.org/repo/gmp/rev/beda46a3c10d

This patch backports "upstream" change (`exit(0)` relaced by
`return 0` in conftest.c)

Signed-off-by: Sergey V. Lobanov <sergey@lobanov.in>
---
 m4/ccforbuild.m4 | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/m4/ccforbuild.m4
+++ b/m4/ccforbuild.m4
@@ -69,7 +69,7 @@ cat >conftest.c <<EOF
 int
 main ()
 {
-  exit(0);
+  return 0;
 }
 EOF
 gmp_compile="$1 conftest.c"
@@ -143,7 +143,7 @@ AC_CACHE_CHECK([for build system executa
 int
 main ()
 {
-  exit (0);
+  return 0;
 }
 EOF
 for i in .exe ,ff8 ""; do