When building giflib on non-Darwin platforms, the all documentation target in the doc Makefile will be built. In order to build the giflib-logo.gif target in the Makefile, ImageMagick must be installed to run the convert command (resize the giflib logo). As a result, since giflib-logo.gif is a second-order dependency of the all target in doc, ImageMagick is required to build giflib on non-Darwin platforms.
This does not seem necessary or intentional as the build instructions only describe ImageMagick as a required build dependency for modifying the giflib website:
If you are going no modify the website, you will also need to have asciidoc and the ImageMagick convert(1) utility installed.
Proposed fix: remove giflib-logo.gif as a prerequisite for the allhtml target and add it as a prerequisite for the website target only.
Please disregard and close this issue as it was already addressed by Avoid having an install-time dependency on convert(1).
Fix pushed.