Add libavcore.

The new library is meant to contain the core multimedia utilities for
FFmpeg, to make them shareable between more libav* libraries.

See thread:
Subject: [FFmpeg-devel] [RFC] New library for shared non-generic libav* utils
Date: Fri, 9 Jul 2010 01:07:40 +0200

Originally committed as revision 24393 to svn://svn.ffmpeg.org/ffmpeg/trunk