Fix backwards compatibility for imagemagick dependencies

In later versions of ImageMagick, the include path changed from ImageMagick/ to ImageMagick-6/. This seems to break several libraries that have portions of this path hard-coded into their configure scripts. Not sure if this is an appropriate way to solve that until such time as all the other stuff out there gets updated. 

My primary use case is PHP's imagick extension, but it seems there may be some ruby gems affected as well.