Can GraphicsMagick become an optional dependency?

[rubyFeedback]

Today I set up my computer anew.

Trying to compile timg quickly I ran into:

The following required packages were not found:

GraphicsMagick++

Ok no problem, I can compile graphicsmagick, but can we make this an
optional dependency? Would timg lose some important functionality?
(No idea right now, it is a genuine question.)

If timg may still be useful without graphicsmagick, perhaps a configure
flag could be used to enable/disable graphicsmagick. Anyway, I'll next
compile graphicsmagick - please feel free to handle this issue in any
way as you see fit, including non-handling of it. :)

[hzeller]

There is the WITH_GRAPHICSMAGICK flag for cmake which you can use to switch off depending on graphicsmagic

cmake -DWITH_GRAPHICSMAGICK=off ../