Global setting [ghostscript library]?
Looks like ImageMagick was built against the Ghostscript delegate library rather than the Ghostscript delegate program (i.e. gs). To change versions of Ghostscript you will need to rebuild ImageMagick from source against the new Ghostscript libraries or add --without-gslib to the configure program to force ImageMagick to use the gs program rather than the Ghostscript libraries.