ImageMagick is not found on Debian/Ubuntu

[olebole]

Triggered by #1060, I switched the Debian package to use ImageMagick instead of GraphicsMagic, which I did with the CMAKE defines DMAGICK=YES DGRAPHICSMAGIC=OFF.

The ImageMagick configutaion uses Magick++-config to find out the compiler flags and libs; however this executable is hidden in Debian (it is in /usr/lib/x86_64-linux-gnu/ImageMagick-6.9.11/bin-q16/MagickCore-config). Therefore, the configuration fails, and I would not like to specify the full path during the configuration.

However, pkg-config is available here, and by replacing the Magick++-config calls with pkg-config ImageMagick, I could get it compiling. Probably the best would be to try both, but I am not good enough to write this myself in a well-structured way (CMake is still a dark chapter for me). My raw attempt is however here.