gimp from console in slackware64-current , error message
 

execute gimp from konsole..gets this error.

that error should be ignored, see https://bugs.archlinux.org/task/42289.

fedora have more patch for gegl:

maybe one solve this:

http://pkgs.fedoraproject.org/cgit/rpms/gegl.git/tree/

no, they doesn't: fedora passes to the configure --without-libavformat so they don't use ffmpeg at all and they don't need any patches.

IMHO it doesn't even make any sense using the patch linked in the archlinux bug report or a snapshot from the gegl-0-2 branch because more deprecated ffmpeg functions still called in the code (avcodec_alloc_frame, for example) will error in a similar fashion.

if I were you I would just ignore it.