Configurable FFmpeg expression, fdk-aac library, license fixes

[ttuegel]

These commits do several things:

• Add ffmpeg_configurable, a configurable expression for FFmpeg styled after vim_configurable. Otherwise, it's difficult to configure an FFmpeg expression for users who want the command line tool in their environment.
• Add the fdk-aac library, a high-quality AAC codec implementation from Android.
• Allow ffmpeg_2 and ffmpeg_configurable to use the fdk-aac library. This is disabled by default because:
• FFmpeg is GPLv2+ unless fdk-aac or faac are enabled, in which case it is non-free and unredistributable.

[vcunat]

It looks good, thanks!

[vcunat]

Actually, I planned to make a configurable ffmpeg/libav switch, so this will be a very nice base. Also for supporting multiple versions inside one expression. When I get to it... there are higher-priority issues right now (e.g. stdenv-updates).

[edolstra]

Hm, we already have 5 variants of ffmpeg in Nixpkgs, do we really want another one? In particular, why do we need both ffmpeg/2.x.nix and ffmpeg/configurable.nix, given that they both build version 2.1.1?

[domenkozar]

I'd say 2.x.nix should use configurable one.

[vcunat]

I think all should use one configurable expression (a slight modification of this one, for example).

[vcunat]

(So far the files only differed in hash and version AFAIK. And maybe some older versions aren't really needed anymore.)

[bluescreen303]

I recently upgraded every (minor) version and checked each package in
nixpkgs to find the highest supported versions. All versions that are
around now are needed by at least 1 package.

[vcunat]

@bluescreen303: even if considering updating the packages (that don't support newer versions)?

[bluescreen303]

I looked at this and upgraded some.
Especially for packages that depend on the older versions of ffmpeg
(0.6., 0.10.) this was without luck. I tried intermediate releases
(0.8, 0.11, 1.0, 1.1) too. There might be unofficial patches for some
packages and in many cases they might be simple to fix manually, but I
didn't look further than official releases.