diff -Nru flex-2.6.4.orig/configure.ac flex-2.6.4/configure.ac
--- flex-2.6.4.orig/configure.ac	2017-10-16 23:01:23.000000000 +0200
+++ flex-2.6.4/configure.ac	2017-11-28 14:35:03.664347578 +0100
@@ -25,8 +25,10 @@
 # autoconf requirements and initialization
 
 AC_INIT([the fast lexical analyser generator],[2.6.4],[flex-help@lists.sourceforge.net],[flex])
+AC_PREREQ([2.60])
 AC_CONFIG_SRCDIR([src/scan.l])
 AC_CONFIG_AUX_DIR([build-aux])
+AC_USE_SYSTEM_EXTENSIONS
 LT_INIT
 AM_INIT_AUTOMAKE([1.11.3 -Wno-portability foreign check-news std-options dist-lzip parallel-tests subdir-objects])
 AC_CONFIG_HEADER([src/config.h])