emacs: make ImageMagick dependency optional in Emacs≥27
emacs: make ImageMagick dependency optional in Emacs≥27

Emacs 27 doesn't use ImageMagick by default to display images [1]:

    ** Emacs no longer defaults to using ImageMagick to display images.
    This is due to security and stability concerns with ImageMagick.  To
    override the default, use 'configure --with-imagemagick'.

[1] https://github.com/emacs-mirror/emacs/blob/emacs-27.1/etc/NEWS#L96-L98

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