Simplify grammar and lexer files to support other build systems

This change does not effect the current build or runtime, and produces the same output files. The reason for the change is to support other build systems, i.e., CMake, Ninja and Xcode.

Making all in modules
  YACC     codec/webvtt/CSSGrammar.c
  YACC     demux/json/grammar.c
demux/json/grammar.h is unchanged
codec/webvtt/CSSGrammar.h is unchanged

# ...

Making install in .
  LEX      codec/webvtt/CSSLexer.c
lex.css.c is unchanged
  LEX      demux/json/lexicon.c
lex.json.c is unchanged