net: introduce netgo build tag to build a pure Go net package.
Fixes #4078.

R=golang-dev, bradfitz, rsc, iant
CC=golang-dev
https://golang.org/cl/7100050
diff --git a/src/pkg/net/cgo_linux.go b/src/pkg/net/cgo_linux.go
index f6cefa8..693aef0 100644
--- a/src/pkg/net/cgo_linux.go
+++ b/src/pkg/net/cgo_linux.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build cgo,!netgo
+
 package net
 
 /*