flex: Fix segfault of stage1flex when host has glibc 2.26 or newer

When the host uses glibc 2.26 or newer, Flex will try to use the
newly-introduced reallocarray() function, but as it would not define
_GNU_SOURCE a segmentation fault would occur later on due to the
compiler assumming that the function is implicitly defined.

This issue manifests itself due to a crash of "stage1flex" during the
Flex bootstrap:

  ./stage1flex   -o stage1scan.c ./scan.l
  make[2]: *** [Makefile:1725: stage1scan.c] Segmentation fault (core dumped)

This imports the patch from the upstream Git repository, and adds flags
in the .mk file to rebuild the Autotools scripts and support files. Due
to the latter, the patch to disable the documentation is changed so it
modifies the .am file instead.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
[Arnout:
 - Add Adrian's Sob and upstream ref to new patch;
 - Keep patch 1 as patch 1;
 - Keep Vicente as author of path 1;
 - Add reason for autoreconf in a comment.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/flex/0001-flex-disable-documentation.patch

@@ -1,4 +1,7 @@
-flex: disable documentation
+From a98443c6a72d5821d4e0152311e5c3eb4cdff25c Mon Sep 17 00:00:00 2001
+From: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
+Date: Wed, 13 Sep 2017 17:26:48 +0300
+Subject: [PATCH] flex: disable documentation
 
 Since we prevent the flex binary to be built, we also need to prevent
 the documentation to be built, otherwise it will fail like this:
@@ -9,11 +12,17 @@ make[2]: *** No rule to make target '../src/flex', needed by 'flex.1'.
 Stop.
 
 Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
+[Adrian: patch Makefile.am instead of Makefile.in]
+Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
+---
+ Makefile.am | 1 -
+ 1 file changed, 1 deletion(-)
 
-diff -rup a/Makefile.in b/Makefile.in
---- a/Makefile.in	2017-05-06 15:49:09.000000000 +0100
-+++ b/Makefile.in	2017-05-10 14:35:20.152794902 +0100
-@@ -450,7 +450,6 @@ EXTRA_DIST = \
+diff --git a/Makefile.am b/Makefile.am
+index 638c549..9db3420 100644
+--- a/Makefile.am
++++ b/Makefile.am
+@@ -43,7 +43,6 @@ EXTRA_DIST = \
  
  SUBDIRS = \
  	src \
@@ -21,3 +30,6 @@ diff -rup a/Makefile.in b/Makefile.in
  	examples \
  	po \
  	tests \
+-- 
+2.14.1
+

package/flex/0002-build-AC_USE_SYSTEM_EXTENSIONS-in-configure.ac.patch

@@ -0,0 +1,35 @@
+From ffa886a580929f26fd5e5a40c9c5334955c48553 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.
+
+Backported from: 24fd0551333e7eded87b64dd36062da3df2f6380
+Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
+---
+ configure.ac | 2 ++
+ 1 file changed, 2 insertions(+)
+
+diff --git a/configure.ac b/configure.ac
+index 55e774b..c879fe1 100644
+--- 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])
+-- 
+2.14.1
+

package/flex/flex.mk

@@ -10,8 +10,14 @@ FLEX_INSTALL_STAGING = YES
 FLEX_LICENSE = FLEX
 FLEX_LICENSE_FILES = COPYING
 FLEX_DEPENDENCIES = $(TARGET_NLS_DEPENDENCIES) host-m4
+
+# 0001-flex-disable-documentation.patch
+# 0002-build-AC_USE_SYSTEM_EXTENSIONS-in-configure.ac.patch
+FLEX_AUTORECONF = YES
+FLEX_GETTEXTIZE = YES
 FLEX_CONF_ENV = ac_cv_path_M4=/usr/bin/m4 \
 	ac_cv_func_reallocarray=no
+
 HOST_FLEX_DEPENDENCIES = host-m4
 
 define FLEX_DISABLE_PROGRAM