fix for using ImageMagick 7.x installs + autodetect ImageMagick+GhostScript#13

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GerHobbelt wants to merge 1 commit intoitext:developfrom

GerHobbelt:autodetect-tools

Commits on Apr 14, 2020

- fix for using ImageMagick 7.x installs where the compare tool has been migrated into the magick tool itself: https://imagemagick.org/script/porting.php
GerHobbelt
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