No longer use bison is yacc emulation mode.

The autoconf AC_PROG_YACC used in configure sets the YACC variable to "bison -y" but:

• we do not use any feature of the yacc emulation mode, in particular for output file names.
• we use a lot of bison specific features: this gives a lot of warnings with recent bison versions. Note an alternative is to disable yacc mode warnings (-Wno-yacc) but even it requires more testing avoid the yacc mode is a far cleaner solution.