AC_PROG_YACC: don't force Bison to warn against its own features

AC_PROG_YACC: don't force Bison to warn against its own features

When invoked with -y/--yacc, Bison warns when its extensions over
POSIX Yacc are used.  Yet many packages requiring GNU Bison use
Autoconf/Automake's Yacc support, which passes -y to Bison.  It
turns out that passing '-o y.tab.c' has exactly the desired
effect: generating not only y.tab.c but also y.tab.h with -d and
y.output with -v.  See:
http://lists.gnu.org/archive/html/bison-patches/2013-02/msg00100.html
* lib/autoconf/programs.m4 (AC_PROG_YACC): Use bison -o y.tab.c.
* NEWS, doc/autoconf.texi: Document this change.