This deprecates the ImageMagick_VERSION_STRING result variable.
Additionally, ImageMagick version is now determined by either mogrify
executable (if available), or from the `<magick/version.h>` header.
Issue: #27088
- With this change we can use e.g. ImageMagick::Magick++ directly
in targt_link_libraries.
- This change also adds CFLAGS which was missing before.
- Also adds example on how to use the targets.