Unsurprisingly, ffmpeg isn't really designed with cross-compilation in mind
TechNote 2137:
http://developer.apple.com/technotes/tn2005/tn2137.html
This technote seems to assume use of the GNU autotools package
Not all projects use autotools - they're very esoteric much of the time
Basic process is the same as that TN
When we started, ffmpeg didn't even compile for ICBMs
VLC had a patch that would work
Configuring for cross-compiling is not necessarily easy
Next:
Configuring ffmpeg for cross-compilation