switch tounconditionnal boostrapping while to build the tree

[Bruce Evans]

On Sat, 16 Jun 2012, Robert Millan wrote:

> 2012/5/27 Baptiste Daroussin <bapt at freebsd.org>:
>> so if no one care I'll remove the condition to boostrap at least yacc(1) and
>> lex(1) on current, 9, 8 and 7.
>
> While we are at it, would you mind if ${YACC} was set to "byacc" on current?
>
> This would make sys.mk re-usable on systems where "yacc" is a symlink
> to bison (e.g. Debian).

This is probably not permitted in the POSIX case.  POSIX specifies:

% 23836                 Default Rules
% 23837                 The default rules for make shall achieve results that are the same as if the following were used.
% ...
% 23850                 YACC=yacc

If YACC=yacc were used, then it would achieves different results from
YACC=byacc if yacc is a link to bison; therefore, YACC=byacc would be
a bug.

It seems wrong for FreeBSD's sys.mk to support non-BSD systems anyway.

Bruce