You should definitely commit the output of yacc to git. This way not only you don't require your users to have yacc, but you make sure they are using the correct yacc version; you know, the one you tested against. And when you change the yacc version, you'll know because it's in your history; otherwise it's hard to remember at which particular time you decided to upgrade the yacc version used by the project.

Committing cpp output makes no sense because cpp output is specific to the machine which ran cpp.