In devel/rpc2, configure checks for a yacc with AC_PROG_YACC, which
looks for bison -y and byacc and sets YACC to yacc if not found.  Coda's
configure.in then checks ac_cv_PROG_YACC and fails if not set.  This is
buggy, because autoconf doesn't say that it sets that variable if the
fallback program is used, but I'm not sure how to fix.  I think autoconf
should explicitly look for yacc, because there could be systems without
it.

For pkgsrc on NetBSD, base system yacc ought to be good enough, but on
GNU/Linux, there might not be yacc, and we'd have to depend on bison.
So do we need a mk/yacc.mk that pulls in something?  Perhaps we need a
meta-framework for that so we can express this as data rather than
having to write mk files for each program.

For now, I can just add a dependency on bison or patch configure not to
exit.  Does pkgsrc run on systems that might not have yacc?