gpac 0.4.2 uses a variable that doesn't exist anymore in latest ffmpeg version. A patch solving this is following. This is more or less the same probelm as for bug #144989 , so it's more or less the same fix ;)
Created attachment 97944 [details, diff] Patch fixing it
Fixed in cvs, patch applied to ffmpeg_decode.c instead of ffmpeg_in.h in order to leave .h's as close as possible to upstream.