unicode: allow version to be passed by env var

This, in turn, to make it work with x/text’s
go generate.

Also eliminates need to manually update version
string in maketables.go.

Change-Id: Id5a8b8e27bdce5b1b5920eb9223a2d27b889149a
Reviewed-on: https://go-review.googlesource.com/63952
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/unicode/maketables.go

@@ -41,10 +41,17 @@ func main() {
 	flushOutput()
 }
 
+func defaultVersion() string {
+	if v := os.Getenv("UNICODE_VERSION"); v != "" {
+		return v
+	}
+	return unicode.Version
+}
+
 var dataURL = flag.String("data", "", "full URL for UnicodeData.txt; defaults to --url/UnicodeData.txt")
 var casefoldingURL = flag.String("casefolding", "", "full URL for CaseFolding.txt; defaults to --url/CaseFolding.txt")
 var url = flag.String("url",
-	"http://www.unicode.org/Public/9.0.0/ucd/",
+	"http://www.unicode.org/Public/"+defaultVersion()+"/ucd/",
 	"URL of Unicode database directory")
 var tablelist = flag.String("tables",
 	"all",