Updated giflib on macOS to 5.2.2 by radarhere · Pull Request #7815 · python-pillow/Pillow
giflib 5.2.2 has been released - https://sourceforge.net/projects/giflib/files/
Building on Linux, I found that it now requires ImageMagick to build. I created https://sourceforge.net/p/giflib/feature-requests/10/ to inquire further, and it should be fixed in the next version of giflib.
In the meantime though, this PR updates the dependency for macOS only.