maint: fix some syntax-check issues

* cfg.mk: Nuke the following warnings which are confused by our
text reports (that state that the error token is number 256).
  prohibit_magic_number_exit
  ../../doc/bison.texi:8170:error (256)
  ../../tests/conflicts.at:570:error (256)
  ../../tests/conflicts.at:673:error (256)
  ../../tests/conflicts.at:811:error (256)
  ../../tests/conflicts.at:1154:error (256)
  ../../tests/regression.at:281:error (256)
  ../../tests/regression.at:582:error (256)
  maint.mk: use EXIT_* values rather than magic number