Make the icons build step optional.

Make the icons build step optional.

This way, ImageMagick is no longer a hard build dependency. For
developers or users, building puzzles without nice icons is preferable
to not building them at all.

(Also, thanks to Michael Quevillon for pointing out very promptly that
my use of 'REQUIRED' in the find_program command was implicitly
depending on a version of CMake in advance of my minimum_required
specification. This change fixes that too, in passing.)