cf: Restore AC_KRB_PROG_YACC to life

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.

configure.ac

@@ -59,8 +59,8 @@ if ! test -f "$srcdir/lib/asn1/der-protos.h" ||
     AC_KRB_PERL_MOD(JSON)
 fi
 
-dnl AC_KRB_PROG_YACC
-AC_PROG_YACC
+AC_KRB_PROG_YACC
+dnl AC_PROG_YACC
 AM_PROG_LEX
 AS_IF([$LEX --nounput -V > /dev/null 2>&1 && test $? -eq 0],
       [AC_SUBST([FLEXNOUNPUTARGS], ["--nounput"])],