Investigate on ExternalProject for CMake

[Ceylo]

ExternalProject may help at simplifying the configuration and build of FFmpeg. See whether this is usable, with specific care for Windows as it doesn't come presinstalled wih some required unix tools.

See #31 for more details.

[firefly2442]

I downloaded the latest 2.0 version of FFmpeg, but sadly it does not appear that they use CMake for their build system. However, it looks like ExternalProject can run configure scripts. This is promising because we might be able to move the logic and settings from the build_ffmpeg.sh script to the CMake side. This would make things more transparent for the user.

[Ceylo]

Yes, I've no doubt ExternalProject can build FFmpeg on Unixes, but what about Windows? That's the main issue with ExternalProject.

[firefly2442]

Hmm yes, I'm not sure about the best way to handle that on Windows would be...