Include parser files in distribution tarballs

Right now, make dist doesn't include files generated by yacc/bison and lex/flex because of an old commit at f60f46da.

This breaks out of source builds as reported at https://debbugs.gnu.org/cgi/bugreport.cgi?bug=54390.

The automake developers don't think it makes sense to exclude these files from distribution tarballs and I can't spot a reason either. Could that workaround be dropped please?