[mythtv] MythTV and FFmpeg configure

[Peter Bennett]

The process of syncing new versions of FFmpeg into MythTV is very 
cumbersome:

1. Take a new FFmpeg branch and apply all of our customizations to it

2. Take the configure script from the new FFmpeg branch and apply all 
changes since the last FFmpeg sync to our MythTV configure.

Step 2 takes most of the time and is anything but straightforward.

I am working on updating MythTV configure to eliminate step 2 and make 
the MythTV configure invoke the FFmpeg configure as part of its run. I 
have that working and I am testing it on different configurations. I 
have not yet removed all of the excess FFmpeg code from MythTV 
configure. Once I am ready I will test it in the devel/ffmpeg-resync 
branch to make sure that build slaves work with the new configure.

Peter