Fix cpp-from-stdin detection so that it works even if the

compiler silently accepts an empty command line.
Don't bother searching for bison and yacc unless we are going to use them.

src/configure.in

@@ -355,36 +355,40 @@ echo "- setting LDFLAGS=$LDFLAGS"
 dnl Checks for programs.
 AC_PROG_CPP
 
-dnl Check to see what flags are required to get stdin into cpp
+dnl Check to see what flags are required to get stdin into cpp.
+dnl Note that we need to look at the output, since the C compiler might
+dnl not raise an error when no input files are named on the cmd line.
+dnl (This is a somewhat modified version of AC_EGREP_CPP)
+dnl AC_TRY_CPPSTDIN(PATTERN, PROGRAM, [ACTION-IF-FOUND [,
+dnl                 ACTION-IF-NOT-FOUND]])
 AC_DEFUN(AC_TRY_CPPSTDIN,
 	[AC_REQUIRE_CPP()dnl
 	cat > conftest.$ac_ext <<EOF
 [#]line __oline__ "configure"
 #include "confdefs.h"
-[$1]
+[$2]
 EOF
-	ac_try="$ac_cpp $CPPSTDIN <conftest.$ac_ext >/dev/null 2>conftest.out"
-	AC_TRY_EVAL(ac_try)
-	ac_err=`grep -v '^ *+' conftest.out`
-	if test -z "$ac_err"; then
-	  ifelse([$2], , :, [rm -rf conftest*
-	  $2])
-	else
-	  echo "$ac_err" >&AC_FD_CC
-	  echo "configure: failed program was:" >&AC_FD_CC
-	  cat conftest.$ac_ext >&AC_FD_CC
-	ifelse([$3], , , [  rm -rf conftest*
-	  $3
+dnl eval is necessary to expand ac_cpp.
+dnl Ultrix and Pyramid sh refuse to redirect output of eval, so use subshell.
+if (eval "$ac_cpp $CPPSTDIN <conftest.$ac_ext") 2>&AC_FD_CC |
+  grep "$1" >/dev/null 2>&1; then
+  ifelse([$3], , :, [rm -rf conftest*
+  $3])
+ifelse([$4], , , [else
+  rm -rf conftest*
+  $4
 ])dnl
 fi
-rm -f conftest*])
+rm -f conftest*
+])
 
 AC_MSG_CHECKING(how to use cpp with stdin)
 if test -z "$CPPSTDIN"; then
 AC_CACHE_VAL(ac_cv_cpp_stdin,
-	[  CPPSTDIN=""
-	  AC_TRY_CPPSTDIN([#include <assert.h>
-		Syntax Error], , CPPSTDIN="-")
+	[  CPPSTDIN="-"
+	  AC_TRY_CPPSTDIN([^Find Me Or Else],
+[#include <assert.h>
+Find Me Or Else], , CPPSTDIN="")
 	  ac_cv_cpp_stdin="$CPPSTDIN"])
 	  CPPSTDIN="$ac_cv_cpp_stdin"
 else
@@ -510,24 +514,24 @@ dnl (1) We specify in YACC and YFLAGS what we want
 dnl (2) We have bison and we use bison -y
 dnl (3) We have yacc and use it
 
-AC_SUBST(YACC)
-AC_SUBST(YFLAGS)
-AC_PATH_PROG(yacc, yacc)
-AC_PATH_PROG(bison, bison)
 if test -f "$YACC"
 then
 	echo "- Using $YACC $YFLAGS"
-elif test -f "$bison"
-then
+else
+    AC_PATH_PROG(bison, bison)
+    if test -f "$bison"
+    then
 	echo "- Using $bison -y $YFLAGS"
 	YACC="$bison"
 	YFLAGS="-y $YFLAGS"
-	export YACC YFLAGS
-else
+    else
+	AC_PATH_PROG(yacc, yacc)
 	echo "- Using $yacc $YFLAGS"
 	YACC="$yacc"
-	export YACC
+    fi
 fi
+AC_SUBST(YACC)
+AC_SUBST(YFLAGS)
 
 AC_CHECK_LIB(sfio,     main)
 for curses in ncurses curses ; do