Log Message:
ffmpeg[2-6]: Fixes for Darwin/powerpc.

- Do not universally --enable-rpath (it might not be supported)

- Limit nasm dependency to x86.

- Do not enable X11 as a default option.
  This isn't technically a fix, just a sensemaking improvement.
  X11 support in ffmpeg is used to capture video from an X display,
  Darwin users are unlikely to be running an X server as their primary
  display.