https://bugs.gentoo.org/881709 (and dupe https://bugs.gentoo.org/900066)
https://github.com/proftpd/proftpd/pull/1667

From 98e46de3093da71121362d5be4c445a3ef227010 Mon Sep 17 00:00:00 2001
From: Arjun Shankar <arjun@redhat.com>
Date: Fri, 5 May 2023 15:35:56 +0200
Subject: [PATCH] configure: Remove several implicit function declarations

During configure, some checks omit the corresponding include. A compiler
defaulting to C99 mode could cause those checks to fail since C99 does
not allow implicit function declarations. This commit fixes the same.
The configure script is re-generated.

Signed-off-by: Arjun Shankar <arjun@redhat.com>
--- a/configure
+++ b/configure
@@ -18305,6 +18305,20 @@ _ACEOF
 fi
 
 
+for ac_header in stddef.h crypt.h netdb.h arpa/inet.h sys/socket.h
+do :
+  as_ac_Header=`$as_echo "ac_cv_header_$ac_header" | $as_tr_sh`
+ac_fn_c_check_header_mongrel "$LINENO" "$ac_header" "$as_ac_Header" "$ac_includes_default"
+if eval test \"x\$"$as_ac_Header"\" = x"yes"; then :
+  cat >>confdefs.h <<_ACEOF
+#define `$as_echo "HAVE_$ac_header" | $as_tr_cpp` 1
+_ACEOF
+
+fi
+
+done
+
+
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for standalone crypt" >&5
 $as_echo_n "checking for standalone crypt... " >&6; }
 if ${pr_cv_lib_standalone_crypt+:} false; then :
@@ -18313,10 +18327,19 @@ else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 
+      #if HAVE_STDDEF_H
+      # include <stddef.h>
+      #endif
+      #if HAVE_CRYPT_H
+      # include <crypt.h>
+      #endif
+
 int
 main ()
 {
-crypt();
+
+      crypt(NULL, NULL);
+
   ;
   return 0;
 }
@@ -18388,10 +18411,19 @@ else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 
+      #if HAVE_STDDEF_H
+      # include <stddef.h>
+      #endif
+      #if HAVE_NETDB_H
+      # include <netdb.h>
+      #endif
+
 int
 main ()
 {
-gethostbyname();
+
+      gethostbyname(NULL);
+
   ;
   return 0;
 }
@@ -18508,10 +18540,19 @@ else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 
+      #if HAVE_STDDEF_H
+      # include <stddef.h>
+      #endif
+      #if HAVE_ARPA_INET_H
+      # include <arpa/inet.h>
+      #endif
+
 int
 main ()
 {
-inet_aton();
+
+      inet_aton(NULL, NULL);
+
   ;
   return 0;
 }
@@ -18584,10 +18625,16 @@ else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 
+      #if HAVE_NETDB_H
+      # include <netdb.h>
+      #endif
+
 int
 main ()
 {
-gethostent();
+
+      gethostent();
+
   ;
   return 0;
 }
@@ -18659,10 +18706,19 @@ else
   cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */
 
+      #if HAVE_STDDEF_H
+      # include <stddef.h>
+      #endif
+      #if HAVE_SYS_SOCKET_H
+      # include <sys/socket.h>
+      #endif
+
 int
 main ()
 {
-bind();
+
+      bind(0, NULL, 0);
+
   ;
   return 0;
 }
--- a/configure.in
+++ b/configure.in
@@ -1476,18 +1476,42 @@ AC_ARG_ENABLE(transfer-buffer-size,
   ])
 
 dnl Checks for libraries.  Yes, this is the hard way, but it's necessary.
+AC_CHECK_HEADERS(stddef.h crypt.h netdb.h arpa/inet.h sys/socket.h)
+
 AC_CACHE_CHECK(for standalone crypt,pr_cv_lib_standalone_crypt,
-  AC_TRY_LINK(,[crypt();],
-  	pr_cv_lib_standalone_crypt="yes", pr_cv_lib_standalone_crypt="no" ))
+  AC_TRY_LINK(
+    [
+      #if HAVE_STDDEF_H
+      # include <stddef.h>
+      #endif
+      #if HAVE_CRYPT_H
+      # include <crypt.h>
+      #endif
+    ],
+    [
+      crypt(NULL, NULL);
+    ],
+    pr_cv_lib_standalone_crypt="yes", pr_cv_lib_standalone_crypt="no" ))
 
 if test "$pr_cv_lib_standalone_crypt" = "no"; then
   AC_CHECK_LIB(crypt, crypt)
 fi
 
 AC_CACHE_CHECK(for standalone gethostbyname,pr_cv_lib_standalone_gethost,
-  AC_TRY_LINK(,[gethostbyname();],
-  	pr_cv_lib_standalone_gethost="yes",
-	pr_cv_lib_standalone_gethost="no" ))
+  AC_TRY_LINK(
+    [
+      #if HAVE_STDDEF_H
+      # include <stddef.h>
+      #endif
+      #if HAVE_NETDB_H
+      # include <netdb.h>
+      #endif
+    ],
+    [
+      gethostbyname(NULL);
+    ],
+    pr_cv_lib_standalone_gethost="yes",
+    pr_cv_lib_standalone_gethost="no" ))
 
 if test "$pr_cv_lib_standalone_gethost" = "no"; then
   AC_CHECK_LIB(resolv, gethostbyname)
@@ -1495,25 +1519,57 @@ if test "$pr_cv_lib_standalone_gethost" = "no"; then
 fi
 
 AC_CACHE_CHECK(for standalone inet_aton,pr_cv_lib_standalone_aton,
-  AC_TRY_LINK(,[inet_aton();],
-  	pr_cv_lib_standalone_aton="yes",
-	pr_cv_lib_standalone_aton="no" ))
+  AC_TRY_LINK(
+    [
+      #if HAVE_STDDEF_H
+      # include <stddef.h>
+      #endif
+      #if HAVE_ARPA_INET_H
+      # include <arpa/inet.h>
+      #endif
+    ],
+    [
+      inet_aton(NULL, NULL);
+    ],
+    pr_cv_lib_standalone_aton="yes",
+    pr_cv_lib_standalone_aton="no" ))
 
 if test "$pr_cv_lib_standalone_aton" = "no"; then
   AC_CHECK_LIB(bind, inet_aton)
 fi
 
 AC_CACHE_CHECK(for standalone nsl functions,pr_cv_lib_standalone_nsl,[
-  AC_TRY_LINK(,[gethostent();],
-  pr_cv_lib_standalone_nsl="yes", pr_cv_lib_standalone_nsl="no") ])
+  AC_TRY_LINK(
+    [
+      #if HAVE_NETDB_H
+      # include <netdb.h>
+      #endif
+    ],
+    [
+      gethostent();
+    ],
+    pr_cv_lib_standalone_nsl="yes",
+    pr_cv_lib_standalone_nsl="no") ])
 
 if test "$pr_cv_lib_standalone_nsl" = "no"; then
   AC_CHECK_LIB(nsl, gethostent)
 fi
 
 AC_CACHE_CHECK(for standalone socket functions,pr_cv_lib_standalone_sockets,
-  AC_TRY_LINK(,[bind();],
-  pr_cv_lib_standalone_sockets="yes", pr_cv_lib_standalone_sockets="no"))
+  AC_TRY_LINK(
+    [
+      #if HAVE_STDDEF_H
+      # include <stddef.h>
+      #endif
+      #if HAVE_SYS_SOCKET_H
+      # include <sys/socket.h>
+      #endif
+    ],
+    [
+      bind(0, NULL, 0);
+    ],
+    pr_cv_lib_standalone_sockets="yes",
+    pr_cv_lib_standalone_sockets="no"))
 
 if test "$pr_cv_lib_standalone_sockets" = "no"; then
   AC_CHECK_LIB(socket, bind)