Clean webvtt-config.h.

According to the examples in the automake manual, BUILD_SOURCES
resolves the dependency generation race, but doesn't actualy add
the built sources to the clean file list. We need to do this
explicitly to make sure it's removed from the tree on 'clean'.

This commit has 'make distcheck' working for me for the first time.
Make webvtt-config.h depend on Makefile.

This way it gets remade if the generating rule might have changed.