FindImageMagick: Search quantum depth suffixes (#13859)

From ImageMagick's 6.8.0-8 changelog:

  http://www.imagemagick.org/script/changelog.php
  ABI is incompatible if quantum depth change.
  Add abi indication to library name.

Search for library names with -Q16 and -Q8 suffixes.

Reported-by: Evangelos Foutras <evangelos@foutrelis.com>
Merge topic 'FindImageMagick-v6.8.0-8' into next

5fe9855 FindImageMagick: Search quantum depth suffixes (#13859)