go.crypto/sha3: use the go1.4 tag to register the hash functions.

Prior to Go 1.4, crypto didn't have the consts defined for SHA-3.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/133540043

sha3/hashes.go

@@ -9,17 +9,9 @@ package sha3
 // bytes.
 
 import (
-	"crypto"
 	"hash"
 )
 
-func init() {
-	crypto.RegisterHash(crypto.SHA3_224, New224)
-	crypto.RegisterHash(crypto.SHA3_256, New256)
-	crypto.RegisterHash(crypto.SHA3_384, New384)
-	crypto.RegisterHash(crypto.SHA3_512, New512)
-}
-
 // New224 creates a new SHA3-224 hash.
 // Its generic security strength is 224 bits against preimage attacks,
 // and 112 bits against collision attacks.

sha3/register.go

@@ -0,0 +1,18 @@
+// Copyright 2014 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.
+
+// +build go1.4
+
+package sha3
+
+import (
+	"crypto"
+)
+
+func init() {
+	crypto.RegisterHash(crypto.SHA3_224, New224)
+	crypto.RegisterHash(crypto.SHA3_256, New256)
+	crypto.RegisterHash(crypto.SHA3_384, New384)
+	crypto.RegisterHash(crypto.SHA3_512, New512)
+}