bug#16754: unnecessary requirement of X11 for imagemagick in case of cygwin

[Mirek Kaim <mirek.kaim@outlook.com>]

> But image-dired does not use the built-in ImageMagick support, it just
> calls the external ImageMagick executables. And DocView may do the same
> too.

i'm
 not sure if doc-view-scale-internally implies imagemagick, but what i 
do know is that before recompiling emacs with imagemagick support, 
doc-view displayed the pages at the original resolution (depending on 
dpi setting, obviously), while afterwards it started using the 
doc-view-image-width variable, so my guess is that it works (both builds
 were done by me, the only difference was adding imagemagick and dbus 
support).

of course i may be wrong, so any way to check if it works is welcome.