https://github.com/fvwmorg/fvwm/pull/100

From 0b4daddf6b88b696daf54714448b8d89a615abf2 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Thu, 24 Nov 2022 13:06:50 +0100
Subject: [PATCH 1/2] configure: Do not require support for implicit ints

Implicit ints  have not been part of C since 1999, and future
compilers will disable support for them by default.  Fortunatenly,
only one configure check needs adjusting.
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -30,7 +30,7 @@ if test "$ac_cv_func_select" = yes; then
 #ifdef HAVE_SYS_SOCKET_H
 #include <sys/socket.h>
 #endif],
-[extern select ($ac_cv_type_fd_set_size_t,
+[extern int select ($ac_cv_type_fd_set_size_t,
  $ac_cv_type_fd_set *,	$ac_cv_type_fd_set *, $ac_cv_type_fd_set *,
  $ac_type_timeval *);],
 [ac_found=yes ; break 3],ac_found=no)
--- a/configure.ac
+++ b/configure.ac
@@ -220,7 +220,7 @@ AC_MINIX
 
 # catch -Werror and similar options when running configure
 AC_TRY_COMPILE([#include <stdio.h>],
-[int i; static j; int *p; char *c;
+[int i; int *p; char *c;
   switch (*p = p = *c) { case 0: printf("%Q", c, p); }
   *c = &i; c = p;
   while (1 || (unsigned int)3 >= 0 || ((int)-1) == ((unsigned int)1));

From 6b9d100ae4e784821b5189474dc58e646417bcf6 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Thu, 24 Nov 2022 13:07:56 +0100
Subject: [PATCH 2/2] acinclude.m4: Add missing <unistd.h> to
 AM_SAFETY_CHECK_MKSTEMP

Otherwise, the check fails on a compiler which does not support
implicit function declarations (a language feature removed in 1999).
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -1154,6 +1154,9 @@ AC_DEFUN([AM_SAFETY_CHECK_MKSTEMP],[
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
 int main(void)
 {
   char template[128];