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

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>