Port override skeleton for ffmpeg

[justin at shiningsilence.com]

> 	I couldn't build it without the override (on 1.1-stable from
> a few days ago) because the bktr and meteor headers have moved from
> the FreeBSD 4.x location (machine/) to dev/video/bktr and dev/video/meteor
> and thus weren't found by the compile. Do you still have FreeBSD 4.x
> header
> files lurking ?

I shouldn't; this machine has only ever run DragonFly.

I don't have my brooktree card in this machine, and I'm not setting any
bktr-specific options when compiling, so it may just not be bothering to
look for those files.  I assume you've tested it on a DragonFly machine
with a bktr card inside?