The attached patch is a 1st stab at making it possible to use the
imagemagick library to load images in emacs.

I can load GIMP XCF and DJVU files like this:
src/emacs -q --eval "(insert-image (create-image \"/tmp/tst.xcf\" 'imagemagick) )"

It works with any image format supported by imagemagick, find out which
with:
identify -list format

This patch does not replace the current image loading code, it merely
adds imagemagick as a possible file loader.

(The 1st part of the patch are merely changes to "configure", ignore
those please. The patch was generated with "git diff imagemagick master
>imagemagick1.patch" and configure is a managed file for some reason.)