bug#33587: [PROPOSED] Default to disabling ImageMagick

[Eli Zaretskii]

> From: Paul Eggert <eggert@cs.ucla.edu>
> Date: Sun,  2 Dec 2018 10:09:19 -0800
> Cc: Paul Eggert <eggert@Penguin.CS.UCLA.EDU>
> 
> From: Paul Eggert <eggert@Penguin.CS.UCLA.EDU>
> 
> ImageMagick has continuing stability and security problems, suggesting
> that 'configure' should disable it by default.  See Glenn Morris's notes
> at: https://lists.gnu.org/r/emacs-devel/2018-12/msg00036.html
> * INSTALL, etc/NEWS: Mention this.
> * configure.ac (imagemagick): Default to off.

No objections from me, but let's please wait for a week, to let people
chance to voice objections.

Thanks.