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

doc/go1.2.txt

@@ -15,4 +15,5 @@ crypto/tls: add support for TLS 1.1. (CL 7872043).
 fmt: indexed access to arguments in Printf etc. (CL 9680043).
 go/build: support including C++ code with cgo (CL 8248043).
 io: Copy prioritizes WriterTo over ReaderFrom (CL 9462044).
+net: new build tag netgo for building a pure Go net package (CL 7100050).
 testing: AllocsPerRun is now quantized to an integer (the type is still float64) (CL 9837049).

src/pkg/net/cgo_bsd.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !netgo
 // +build darwin freebsd
 
 package net

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
 
 /*

src/pkg/net/cgo_netbsd.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
 
 /*

src/pkg/net/cgo_openbsd.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
 
 /*

src/pkg/net/cgo_stub.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !cgo
+// +build !cgo netgo
 
 // Stub cgo routines for systems that do not use cgo to do network lookups.
 

src/pkg/net/cgo_unix.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build !netgo
 // +build darwin freebsd linux netbsd openbsd
 
 package net