Remove SDK reference from libav build script

- SDKs are always disappearing (like 10.7 is from latest
  Xcode versions). Obviously these weren't useful anyway
  since they didn't fail even though the SDK is gone.
- Also made minimum OS X version into a variable so it's
  easier to update. Would be best to piggyback on the
  Xcode setting though.
make libav build script use MACOSX_DEPLOYMENT_TARGET

- Xcode sends this down as an environment var.
- Now we only need to update this in one place to change
  the minimum version.