Using `AC_PROG_YACC` means accepting that `bison` gets invoked in yacc
compatibility mode (i.e., with the `-y` command-line option). In recent
versions Bison has started to warn about use of Bison extensions when in
yacc compatibility mode -- even for things yacc itself accepts but which
it doesn't document.
I've confirmed that `bison -d` and `byacc -d` both work for the ASN.1
compiler.
