AC_PROG_YACC question

[David Coquil]

Hi list,

Here's what autoconf (2.13) docs say about AC_PROG_YACC :

        "If bison is found, set output variable YACC to `bison -y'.
         Otherwise, if byacc is found, set YACC to `byacc'.  Otherwise set
         YACC to `yacc'."

I was wondering, if it finds neither bison nor byacc, why doesn't the macro
also looks for yacc before setting the variable ? I'd expect it to be the
job of the configure script to tell the user that the package won't build
unless he installs the package rather than having make fail with some 
`yacc : not found' message. (same for AC_PROG_LEX)

I understand that was probably a design decision, a feature rather than a
bug, but I don't quite get the reasoning behind this decision.

Thanks,

--
David Coquil