giflib 5.0.4

Closes #121.

Signed-off-by: Xiyue Deng <manphiz@gmail.com>
More giflib5 tweaks.

* Make it keg_only to avoid conflicts with giflib in main repository.
* Drop unrecognized "--disable-debug" configure option.

Signed-off-by: Xiyue Deng <manphiz@gmail.com>