FindImageMagick: Fix ARCH_INCLUDE_DIR detection for version 7 and newer

Timo Röhlingrequested to merge
roehling/cmake:image-magick-7 into master

The magick-baseconfig.h header has been moved to a different subdirectory.

See https://bugs.debian.org/1087233 for the original bug report in Debian.

Backport: release

Edited by Brad King

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