Apparently the changes from 3c9185bf
aren't enough; even with that in place, I got errors like this
when trying to build for iOS:

src/libavdevice/avfoundation.m:135:5: error:
      'AVCaptureDeviceTransportControlsPlaybackMode' is unavailable: not
      available on iOS
    AVCaptureDeviceTransportControlsPlaybackMode observed_mode;
    ^