proof of concept for using pkg-config in place of Magick-config on debian based systems

This is admittedly a pretty ugly patch for falling back to using pkg-config instead of Magick-config on newer Debian (and probably many more) systems.

I can't find a good description for what happened to Magick-config in Debian, but it's moved it's way into https://packages.debian.org/sid/graphicsmagick-libmagick-dev-compat which doesn't seem to reference the relevant headers needed for comp (ie: wand/MagickWand.h).

After spending a few days tracking this down, I stumbled across: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=761927 which seems to indicate that Magick-config is going away. Since it's just a pretty simple wrapper around pkg-config (at least on Debian anyway), this patch adds support for compilation on newer Debian systems (jessie/sid) without requiring Magick-config.

It probably makes sense to just get rid of Magick-config all together and port over to pkg-config if the situation on Debian is similar on other OS's.
@prognostikios thanks for catching that, silly mistake on my part.  Magick-config just uses --cflags, so I've updated to that.