I have attached a patch which adds support for ImageMagick version 7.
I have tried to keep compatibility with version 6 as well. I have
gotten clean builds against both versions on my machine (running Arch
and overriding PKG_CONFIG_PATH to test). I also did a quick build on a
Mac and it also seemed to link up to the ImageMagick 7 from homebrew.
On both machines I could open/resize/rotate images and page through
the frames of animated GIFs.

The changes that seem to be needed are doable with a #define and a
typedef to rename a function and type in image.c. The patch makes
these conditional on the major version that is found by pkg-config
during ./configure, trying version 7 first and falling back to 6.

I'm not very familiar with autoconf, but the conditional checking in
the patch seems to work and avoids checking for ImageMagick 6 if it
finds version 7 first. I used AS_IF for this rather than the action
arguments to EMACS_CHECK_MODULES since this way seems to avoid the
redundant checks.

Thanks,
Karl