version: add support for setting version with the -X Go linker flag

Updates tailscale/tailscale#486

Signed-off-by: Elias Naur <mail@eliasnaur.com>

version/version.go

@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !redo
+// +build !redo,!xversion
 
 // Package version provides the version that the binary was built at.
 package version
 
-const LONG = "date.20200618"
+const LONG = "date.20200622"
 const SHORT = LONG

version/xversion.go

@@ -0,0 +1,11 @@
+// Copyright (c) 2020 Tailscale Inc & 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 !redo,xversion
+
+package version
+
+// Replaced at build time with the Go linker flag -X.
+var LONG string = "<not set>"
+var SHORT string = "<not set>"