Fix check for yacc/bison in configure script.

configure

@@ -651,6 +651,7 @@ INSTALL_RULE
 ENGINE_C
 YFLAGS
 YACC
+BISON
 SHPROG
 RMPROG
 LN_S
@@ -6536,7 +6537,59 @@ done
 test -n "$SHPROG" || SHPROG="/bin/sh"
 
 
-for ac_prog in 'bison -y' byacc
+for ac_prog in bison
+do
+  # Extract the first word of "$ac_prog", so it can be a program name with args.
+set dummy $ac_prog; ac_word=$2
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for $ac_word" >&5
+printf %s "checking for $ac_word... " >&6; }
+if test ${ac_cv_prog_BISON+y}
+then :
+  printf %s "(cached) " >&6
+else case e in #(
+  e) if test -n "$BISON"; then
+  ac_cv_prog_BISON="$BISON" # Let the user override the test.
+else
+as_save_IFS=$IFS; IFS=$PATH_SEPARATOR
+for as_dir in $PATH
+do
+  IFS=$as_save_IFS
+  case $as_dir in #(((
+    '') as_dir=./ ;;
+    */) ;;
+    *) as_dir=$as_dir/ ;;
+  esac
+    for ac_exec_ext in '' $ac_executable_extensions; do
+  if as_fn_executable_p "$as_dir$ac_word$ac_exec_ext"; then
+    ac_cv_prog_BISON="$ac_prog"
+    printf "%s\n" "$as_me:${as_lineno-$LINENO}: found $as_dir$ac_word$ac_exec_ext" >&5
+    break 2
+  fi
+done
+  done
+IFS=$as_save_IFS
+
+fi ;;
+esac
+fi
+BISON=$ac_cv_prog_BISON
+if test -n "$BISON"; then
+  { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $BISON" >&5
+printf "%s\n" "$BISON" >&6; }
+else
+  { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: no" >&5
+printf "%s\n" "no" >&6; }
+fi
+
+
+  test -n "$BISON" && break
+done
+
+
+if test -n "$BISON"; then
+  YACC="$BISON -y"
+else
+  for ac_prog in 'bison -y' byacc
 do
   # Extract the first word of "$ac_prog", so it can be a program name with args.
 set dummy $ac_prog; ac_word=$2
@@ -6585,13 +6638,35 @@ fi
 done
 test -n "$YACC" || YACC="yacc"
 
-if test "$YACC" = ":" ; then
-  as_fn_error $? "Cannot find yacc." "$LINENO" 5
-elif echo "" | $YACC -V -v --version > /dev/null 2>&1 ; then
-  :
-else
-  { printf "%s\n" "$as_me:${as_lineno-$LINENO}: WARNING: $YACC may not work as yacc." >&5
-printf "%s\n" "$as_me: WARNING: $YACC may not work as yacc." >&2;}
+fi
+
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether $YACC can parse a minimal grammar" >&5
+printf %s "checking whether $YACC can parse a minimal grammar... " >&6; }
+if test ${ac_cv_yacc_works+y}
+then :
+  printf %s "(cached) " >&6
+else case e in #(
+  e)
+  cat > conftest.y <<'EOF'
+%token T
+%%
+start: /* empty */ ;
+%%
+EOF
+  if $YACC -d conftest.y >/dev/null 2>&1; then
+    ac_cv_yacc_works=yes
+  else
+    ac_cv_yacc_works=no
+  fi
+  rm -f conftest.y conftest.tab.c conftest.tab.h conftest.output
+ ;;
+esac
+fi
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $ac_cv_yacc_works" >&5
+printf "%s\n" "$ac_cv_yacc_works" >&6; }
+
+if test "$ac_cv_yacc_works" = "no"; then
+  as_fn_error $? "No working yacc/bison found. Please install bison or a compatible yacc (e.g. byacc)." "$LINENO" 5
 fi
 
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking for posix non-blocking" >&5

configure.ac

@@ -125,16 +125,33 @@ AC_PATH_PROGS(RMPROG, rm, /bin/rm)
 AC_PATH_PROGS(SHPROG, sh, /bin/sh)
 
 dnl YACC - ditto
-AC_PROG_YACC
-dnl The autoconf docs say $YACC defaults to 'yacc'.  This seems to be true,
-dnl but judging from AC_PROG_LEX, it may not stay true.
-if test "$YACC" = ":" ; then
-  AC_MSG_ERROR([Cannot find yacc.])
-elif echo "" | $YACC -V -v --version > /dev/null 2>&1 ; then
-  :
+dnl Prefer bison if present
+AC_CHECK_PROGS([BISON], [bison])
+
+if test -n "$BISON"; then
+  YACC="$BISON -y"
 else
-dnl byacc does not seem to have any way to test for workingness, so only warn.
-  AC_MSG_WARN([$YACC may not work as yacc.])
+  AC_PROG_YACC
+fi
+
+dnl Test whether the selected $YACC actually works
+AC_CACHE_CHECK([whether $YACC can parse a minimal grammar], [ac_cv_yacc_works], [
+  cat > conftest.y <<'EOF'
+%token T
+%%
+start: /* empty */ ;
+%%
+EOF
+  if $YACC -d conftest.y >/dev/null 2>&1; then
+    ac_cv_yacc_works=yes
+  else
+    ac_cv_yacc_works=no
+  fi
+  rm -f conftest.y conftest.tab.c conftest.tab.h conftest.output
+])
+
+if test "$ac_cv_yacc_works" = "no"; then
+  AC_MSG_ERROR([No working yacc/bison found. Please install bison or a compatible yacc (e.g. byacc).])
 fi
 
 unet_NONBLOCKING