diff --git a/configure.ac b/configure.ac
index e42667ddf..33e939ae4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,13 +1,13 @@
 dnl configure.ac: this file is processed by autoconf to produce ./configure.
 
-AC_PREREQ(2.61)
+AC_PREREQ([2.71])
 AC_INIT([Eggdrop],[1.9.5],[bugs@eggheads.org])
 AC_COPYRIGHT([Copyright (C) 1999 - 2023 Eggheads Development Team])
 AC_LANG([C])
 AC_REVISION([m4_esyscmd([misc/getcommit])])
 AC_CONFIG_SRCDIR(src/eggdrop.h)
 AC_CONFIG_AUX_DIR(misc)
-AC_CONFIG_HEADER(config.h)
+AC_CONFIG_HEADERS([config.h])
 AC_PREFIX_DEFAULT([\${HOME}/eggdrop])
 AC_PRESERVE_HELP_ORDER
 
@@ -43,13 +43,16 @@ EGG_SAVE_PARAMETERS
 # Check for a working C compiler
 AC_PROG_CC([gcc cc clang])
 EGG_CHECK_CC
-AC_PROG_CC_C99
+m4_warn([obsolete],
+[AC_PROG_CC_C99 is obsolete; use AC_PROG_CC
+])dnl
+AC_REQUIRE(AC_PROG_CC)
 EGG_CHECK_CC_C99
 
 # These 3 need to be done before any AC_COMPILE_IFELSE()'s.
-AC_AIX
-AC_ISC_POSIX
-AC_MINIX
+AC_USE_SYSTEM_EXTENSIONS
+AC_SEARCH_LIBS([strerror],[cposix])
+AC_USE_SYSTEM_EXTENSIONS
 
 # Check C compiler characteristics.
 EGG_CHECK_ICC
@@ -92,7 +95,20 @@ EGG_CHECK_OS
 # Checks for header files.
 EGG_HEADER_STDC
 AC_HEADER_DIRENT
-AC_HEADER_TIME
+m4_warn([obsolete],
+[Update your code to rely only on HAVE_SYS_TIME_H,
+then remove this warning and the obsolete code below it.
+All current systems provide time.h; it need not be checked for.
+Not all systems provide sys/time.h, but those that do, all allow
+you to include it and time.h simultaneously.])dnl
+AC_CHECK_HEADERS_ONCE([sys/time.h])
+# Obsolete code to be removed.
+if test $ac_cv_header_sys_time_h = yes; then
+  AC_DEFINE([TIME_WITH_SYS_TIME],[1],[Define to 1 if you can safely include both <sys/time.h>
+	     and <time.h>.  This macro is obsolete.])
+fi
+# End of obsolete code.
+
 AC_CHECK_HEADERS([arpa/inet.h fcntl.h limits.h locale.h netdb.h netinet/in.h stdio.h stdarg.h stddef.h sys/file.h sys/param.h sys/select.h sys/socket.h sys/time.h unistd.h wchar.h])