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

[Jeff Law]

On 11/09/2017 12:28 AM, Thomas Koenig wrote:
> 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>
> 
> Â Â Â Â Â Â Â  PR bootstrap/82856
> Â Â Â Â Â Â Â  doc/install.texi: Document incompatibility of Perl >=5.6.26
> Â Â Â Â Â Â Â  with the required version of automake 1.11.6.
> 
OK.
jeff