Enable ffmpeg's GIF decoder for post r8520 revisions (Many thanks to Baptiste Coudurier for fixing the bug(s)).

Gestures: Fix two dead locks when changing audio track and subtitle track when there is no such track.

(though this seem to only happen with debuggers and (?)broken OSes)

[1] system() blocks SIGCHLD, and is probably not intended for multithreading

We cannot assume that blocking calls in other libraries ignore EINTR anyway, 
so the only safe approach is to block in all threads (you can unblock it in
your thread if you really want it) except the signal handling thread.