Bug#821415: corsix-th: FTBFS against ffmpeg 3.0

[Alexandre Detiste]

> hi, happily sponsored!

thanks for this
 
 
> thanks for the fix!
thanks for that too

> (but I think the dh_auto_clean target can be removed, but I didn't check!)

That's my own contraption,
but I have absolutely no idea why it was needed;
that was when --buildsystem=cmake got added,
maybe there were some autoconf cruft left around.

In doubt, just remove it.