if (! defined $configure_vars{'YACC'})

[Bruce Korb]

What is this?  Maybe not everyone wants to use your
version of AC_PROG_YACC.  I use my own:  AG_PROG_BYACC
because I have a strong preference for BYACC.  But,
your latest automake is making some sort of attempt
to be really rigorous and is sticking its nose into
my business.  This is wrong.  You can *NOT* presume to
know more than me.  automake needs to be a tool that
helps me accomplish my goals, not get in my way.  Grr.