Move to graphicsmagick

[Cyrille Berger]

> The only reason I haven't done so was because I thought graphicsmagick
> wasn't packaged by distributions. If it is, or if we have enough clout to
> get it packaged, I think we should do it like greased lightning.
From irc:
12:02 < isaac> 12:00 < Riddell> developers shouldn't not use something because 
it's not packaged, if a program needs it people will package it unless 
there's some obvious problem with it

and I got flame for telling that there might be a problem, anyway, we might, 
for the moment, keep the imagemagick and mark it as deprecated, and have the 
graphicsmagick be activated over imagemagick if it is available and have a 
configure message to warn the user that imagemagick is deprecated and won't 
be maintained in the future.
-- 
--- Cyrille Berger ---