Build: ffmpeg as third-party library from source.

This patch should hopefully build a GPL3-friendly ffmpeg/libav from source on Linux and Windows, for use with Krita's new Recorder Docker. After this is merged, Eoin and I might be able to rewrite parts of our animation rendering/video saving code to use this internal libav instead of standalone ffmpeg.

We will probably need @dkazakov's help testing on Windows, and @ivany's help to get things set up on Mac, when they have time. :)

I think our build environment will need:

autoconf 
automake 
build-essential 
cmake 
git-core 
libass-dev 
libfreetype6-dev 
libgnutls28-dev 
libsdl2-dev 
libtool 
libva-dev 
libvdpau-dev 
libvorbis-dev 
libxcb1-dev 
libxcb-shm0-dev 
libxcb-xfixes0-dev 
pkg-config 
texinfo 
wget 
yasm 
nasm 
zlib1g-dev
libx264-dev
libx265-dev
libvpx-dev
libopus-dev

(It would also be nice to have some help double checking that we are properly complying with licenses.)

References:

https://trac.ffmpeg.org/wiki/CompilationGuide <-- Build

http://ffmpeg.org/legal.html <-- License