cmd/yacc: expr example support windows

1. expr append executable extension.
2. support '\r' character.

Fixes #6851.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/35330043

src/cmd/yacc/Makefile

@@ -2,9 +2,11 @@
 # Use of this source code is governed by a BSD-style
 # license that can be found in the LICENSE file.
 
-expr: yacc.go expr.y
+TARG=expr$(shell go env GOEXE)
+
+$(TARG): yacc.go expr.y
 	go run yacc.go -p expr expr.y
-	go build -o expr y.go
+	go build -o $(TARG) y.go
 
 clean:
-	rm -f y.go y.output expr
+	rm -f y.go y.output $(TARG)

src/cmd/yacc/expr.y

@@ -125,7 +125,7 @@ func (x *exprLex) Lex(yylval *exprSymType) int {
 		case '÷':
 			return '/'
 
-		case ' ', '\t', '\n':
+		case ' ', '\t', '\n', '\r':
 		default:
 			log.Printf("unrecognized character %q", c)
 		}