The vod module repackages MP4 files on the fly, so that a single set of
files on disk can be served as HLS, DASH, MSS or HDS without transcoding.

Use cases:
- Serving one MP4 source to players that expect different streaming
  protocols
- Clipping, rate filtering and joining of media on request
- Encryption for AES-128, SAMPLE-AES and common DRM systems
- Generating thumbnails and extracting subtitles from the source files

This port only installs the module sources.  The module itself is built
by the nginx ports when their VOD option is enabled.