As I mentioned last year, ImageMagick has moved onto -7. The -6 version is in maintenance mode. Much of the structure has changed, which appears to mke changing the programs which use it complex, but the headers and libraries themselves can be installed in parallel.
Unfortunately, the main pkgconfig files have the same names, so if you install both versions then the pkgconfig file will be whichever version you installed last.
I've been asked if you guys have any development patches to support ImageMagick-7 ? But maybe I should just ask if you have any plans to eventually support the newer version ?
For the moment, I suspect the answer is no - and after looking at the code I wouldn't achieve anything in a month of sundays. So, for my own purposes I have a cunning plan : sed the Makefile in inkscape to add '6' to all the pkgconfig filenames, and then sed all users (inkscape, possibly some others) to add the 6 to the names. Dunno if I'll manage it, but it looks possible.
ĸen