flex and bison in autoconf

[Rudra Banerjee]

Hello friends,
My configure.ac is:

AC_PREREQ(2.69)
AC_INIT([mkbib], [2.1], address@hidden)
AC_CONFIG_AUX_DIR([build-aux])
AM_INIT_AUTOMAKE([1.9.6 dist-bzip2 subdir-objects foreign])

# Checks for programs.
AC_PROG_CC

AC_PROG_YACC
#if test x"$YACC" != x"yes"; then
#  AC_MSG_ERROR([Please install bison before installing.])
#fi
AC_PROG_LEX
if test "x$LEX" != xflex; then
  AC_MSG_ERROR([Please install flex before installing.])
fi

AC_CONFIG_MACRO_DIR([m4])
GNOME_DOC_INIT 

# Compiling sources with per-target flags requires AM_PROG_CC_C_O
AM_PROG_CC_C_O
AC_PROG_INSTALL

#AC_PROG_LIBTOOL
PKG_CHECK_MODULES(LIBSOUP, libsoup-2.4 >= 2.26)
AC_SUBST(LIBSOUP_CFLAGS)
AC_SUBST(LIBSOUP_LIBS)

AM_PATH_GTK_3_0([3.4.0],,AC_MSG_ERROR([Gtk+ 3.0.0 or higher required.]))

AC_CONFIG_HEADERS([config.h])
AC_CONFIG_FILES([
        Makefile
        help/Makefile
])
AC_OUTPUT

The problem is, while running ./configure built from this file, do check
existence of YACC (as bison, when the 3 line following AC_PROG_YACC, it
is showing unary error), but is not exiting once error is detected. As,
the run, it shows:
checking for bison... no
checking for byacc... no

checking that generated files are newer than configure... done
configure: creating ./config.status
config.status: creating Makefile
config.status: creating help/Makefile
config.status: creating config.h
config.status: config.h is unchanged
config.status: executing depfiles commands

So, how can I make configure.ac to stop for missing bison ?