unicode: use go generate instead of make to create tables.go

LGTM=mpvl, rsc
R=mpvl, rsc
CC=golang-codereviews
https://golang.org/cl/135820043

src/pkg/unicode/Makefile

-# Copyright 2009 The Go Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style
-# license that can be found in the LICENSE file.
-
-maketables: maketables.go
-	go build maketables.go
-
-tables:	maketables
-	./maketables --tables=all -output tables.go
-
-# Downloads from www.unicode.org, so not part
-# of standard test scripts.
-testtables: maketables
-	@echo '***' Be sure to make tables and go install first
-	./maketables -test

src/pkg/unicode/letter.go

@@ -6,6 +6,9 @@
 // Unicode code points.
 package unicode
 
+// Tables are regenerated each time we update the Unicode version.
+//go:generate go run maketables.go -tables=all -output tables.go
+
 const (
 	MaxRune         = '\U0010FFFF' // Maximum valid Unicode code point.
 	ReplacementChar = '\uFFFD'     // Represents invalid code points.