[patch, doc] Document that new Perl version breaks required automake

[Thomas Koenig <tkoenig at netcologne dot de>]

Hello world,

while PR 82856 remains unsolved so far, this documentation patch at
least points people into the right direction if --enable-maintainer-mode
fails due to the incompatibility of the latest Perl version with
the required automkake version.

Tested with "make dvi" and "make pdf". OK for trunk?

2017-11-09  Thomas Koenig  <tkoenig@gcc.gnu.org>

        PR bootstrap/82856
        doc/install.texi: Document incompatibility of Perl >=5.6.26
        with the required version of automake 1.11.6.

Index: doc/install.texi
===================================================================
--- doc/install.texi	(Revision 254408)
+++ doc/install.texi	(Arbeitskopie)
@@ -324,7 +324,7 @@ Necessary (only on some platforms) to untar the so
 systems' @command{tar} programs will also work, only try GNU
 @command{tar} if you have problems.
 
-@item Perl version 5.6.1 (or later)
+@item Perl version between 5.6.1 and 5.6.24
 
 Necessary when targeting Darwin, building @samp{libstdc++},
 and not using @option{--disable-symvers}.
@@ -338,6 +338,8 @@ Necessary when generating manpages from Texinfo ma
 Used by various scripts to generate some files included in SVN (mainly
 Unicode-related and rarely changing) from source tables.
 
+Used by @command{automake}.
+
 @end table
 
 Several support libraries are necessary to build GCC, some are required,
@@ -420,6 +422,9 @@ the 1.11 series, which is currently 1.11.6.  When
 to a newer version, please update all the directories using an older 1.11
 to the latest released version.
 
+Note that @command{automake} 1.11.6 is incompatible with
+@command{perl} version 5.6.26.
+
 @item gettext version 0.14.5 (or later)
 
 Needed to regenerate @file{gcc.pot}.