tools: flex: fix segfault with glibc 2.26+

Fix segmentation fault caused by implicit declaration of function 'reallocarray'. Added patch will enable
reallocarray() prototype in glibc 2.26+ on Linux systems. This fix will be included in flex 2.6.5.

Fixes LEDE issue: FS#1003 (Flex does not build with GCC 7.2)

Signed-off-by: Marko Ratkaj <marko.ratkaj@sartura.hr>

tools/flex/patches/200-build-AC_USE_SYSTEM_EXTENSIONS-in-configure.ac.patch

@@ -0,0 +1,27 @@
+From 24fd0551333e7eded87b64dd36062da3df2f6380 Mon Sep 17 00:00:00 2001
+From: Explorer09 <explorer09@gmail.com>
+Date: Mon, 4 Sep 2017 10:47:33 +0800
+Subject: [PATCH] build: AC_USE_SYSTEM_EXTENSIONS in configure.ac.
+
+This would, e.g. define _GNU_SOURCE in config.h, enabling the
+reallocarray() prototype in glibc 2.26+ on Linux systems with that
+version of glibc.
+
+Fixes #241.
+---
+ configure.ac | 2 ++
+ 1 file changed, 2 insertions(+)
+
+--- a/configure.ac
++++ b/configure.ac
+@@ -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])