These files are from the WebVTT library, and are extracted from rev
6b637c9f30911433e6d5a5be5d8512317a0d8a14 of the git repository at
https://github.com/mozilla/webvtt.

The following CPPFLAGS are used in order to build and link in Mozilla
-DWEBVTT_NO_CONFIG_H=1 -- Prevent inclusion of generated headers
-DWEBVTT_STATIC=1      -- Compile as a static library