Is this using the "prefix:" attribute in the .pc file correctly?

bob

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/usr/include/ImageMagick/Magick++.h

This is non-standard. It can be correct, but I'm guessing this command returns the same thing it does for me:

    $ pkg-config --libs --cflags ImageMagick++
     -lMagick++ -lWand -lMagick  
    $ 

Which basically means the ImageMagick++.pc file is incorrect on your system. I think the more standard installation is to have Magick++.h in /usr/include, but I can understand someone wanting to change it. But that person needs to update the package config script too or compiles will break :)