Require versions of the Libav libraries corresponding to Libav release
0.7. These are:
libavutil 51.7.0
libavcodec 53.5.0
libavformat 53.2.0
libswscale 2.0.0
libpostproc 52.0.0
Also disable the fallback to simple header check if these libraries
could not be found with pkg-config; now compiling without pkg-config
support for these always requires explicitly setting --enable-libav
and any needed compiler/linker flags. The simple check would have let
compilation proceed even if a version mismatch was detected.
Remove unnecessary demuxer.h include from aviheader.h. Through
stheader.h aviheader.h is included in a lot of files. Add missing
mp_msg.h includes to av_sub.c and sd_ass.c (previously hidden by
indirect inclusion through demuxer.h and stream.h).
Do the global initialization of libavcodec and libavformat
(avcodec_register_all(), av_register_all()) immediately on program
startup and remove the initialization calls from various individual
modules that use libavcodec/libavformat functionality.
