diff --git a/ChangeLog b/ChangeLog
index 18c576b5c..b3cd5b8de 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2021-04-09  Oliver Kiddle  <opk@zsh.org>
+
+	* Felipe Contreras: 47785: Src/Modules/files.c, Src/zsh_system.h,
+	Src/mem.c, configure.ac: remove deprecated autoconf functions
+
 2021-04-08  Jun-ichi Takimoto  <takimoto-j@kba.biglobe.ne.jp>
 
 	* 48416: Completion/Unix/Command/_gcore: support macOS, with
diff --git a/Src/Modules/files.c b/Src/Modules/files.c
index 7ebacba6c..a1d6f6bf2 100644
--- a/Src/Modules/files.c
+++ b/Src/Modules/files.c
@@ -32,12 +32,6 @@
 typedef int (*MoveFunc) _((char const *, char const *));
 typedef int (*RecurseFunc) _((char *, char *, struct stat const *, void *));
 
-#ifndef STDC_HEADERS
-extern int link _((const char *, const char *));
-extern int symlink _((const char *, const char *));
-extern int rename _((const char *, const char *));
-#endif
-
 struct recursivecmd;
 
 #include "files.pro"
diff --git a/Src/mem.c b/Src/mem.c
index 5951e57ed..25b2bbce7 100644
--- a/Src/mem.c
+++ b/Src/mem.c
@@ -1072,18 +1072,10 @@ zrealloc(void *ptr, size_t size)
 # endif
 #endif
 
-#if defined(_BSD) && !defined(STDC_HEADERS)
-# define FREE_RET_T   int
-# define FREE_ARG_T   char *
-# define FREE_DO_RET
-# define MALLOC_RET_T char *
-# define MALLOC_ARG_T size_t
-#else
-# define FREE_RET_T   void
-# define FREE_ARG_T   void *
-# define MALLOC_RET_T void *
-# define MALLOC_ARG_T size_t
-#endif
+#define FREE_RET_T   void
+#define FREE_ARG_T   void *
+#define MALLOC_RET_T void *
+#define MALLOC_ARG_T size_t
 
 /* structure for building free list in blocks holding small blocks */
 
diff --git a/Src/zsh_system.h b/Src/zsh_system.h
index 161b073b4..6f4efce96 100644
--- a/Src/zsh_system.h
+++ b/Src/zsh_system.h
@@ -235,16 +235,8 @@ char *alloca _((size_t));
 # include <errno.h>
 #endif
 
-#ifdef TIME_WITH_SYS_TIME
-# include <sys/time.h>
-# include <time.h>
-#else
-# ifdef HAVE_SYS_TIME_H
-# include <sys/time.h>
-# else
-#  include <time.h>
-# endif
-#endif
+#include <sys/time.h>
+#include <time.h>
 
 /* This is needed by some old SCO unices */
 #if !defined(HAVE_STRUCT_TIMEZONE) && !defined(ZSH_OOT_MODULE)
@@ -279,16 +271,7 @@ struct timespec {
 # include <sys/times.h>
 #endif
 
-#if STDC_HEADERS || HAVE_STRING_H
 # include <string.h>
-/* An ANSI string.h and pre-ANSI memory.h might conflict.  */
-# if !STDC_HEADERS && HAVE_MEMORY_H
-#  include <memory.h>
-# endif /* not STDC_HEADERS and HAVE_MEMORY_H */
-#else   /* not STDC_HEADERS and not HAVE_STRING_H */
-# include <strings.h>
-/* memory.h and strings.h conflict on some systems.  */
-#endif /* not STDC_HEADERS and not HAVE_STRING_H */
 
 #ifdef HAVE_LOCALE_H
 # include <locale.h>
diff --git a/configure.ac b/configure.ac
index 16dafac05..e2ddf0e55 100644
--- a/configure.ac
+++ b/configure.ac
@@ -28,7 +28,7 @@ dnl
 AC_INIT
 AC_CONFIG_SRCDIR([Src/zsh.h])
 AC_PREREQ([2.69])
-AC_CONFIG_HEADER(config.h)
+AC_CONFIG_HEADERS([config.h])
 
 dnl What version of zsh are we building ?
 . ${srcdir}/Config/version.mk
@@ -657,8 +657,6 @@ dnl ------------------
 dnl CHECK HEADER FILES
 dnl ------------------
 AC_HEADER_DIRENT
-AC_HEADER_STDC
-AC_HEADER_TIME
 AC_HEADER_STAT
 AC_HEADER_SYS_WAIT