Add cross-compilation Makefile targets and tar-based releases.

Revamp the build system to be more inline with other Prometheus exporters.
Notably add Darwin and Windows build targets, and add support for releases
using tar files.

tools/vendor/github.com/dnephin/govet/rangeloop.go

@@ -0,0 +1,74 @@
+// Copyright 2012 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.
+
+/*
+This file contains the code to check range loop variables bound inside function
+literals that are deferred or launched in new goroutines. We only check
+instances where the defer or go statement is the last statement in the loop
+body, as otherwise we would need whole program analysis.
+
+For example:
+
+	for i, v := range s {
+		go func() {
+			println(i, v) // not what you might expect
+		}()
+	}
+
+See: https://golang.org/doc/go_faq.html#closures_and_goroutines
+*/
+
+package main
+
+import "go/ast"
+
+func init() {
+	register("rangeloops",
+		"check that range loop variables are used correctly",
+		checkRangeLoop,
+		rangeStmt)
+}
+
+// checkRangeLoop walks the body of the provided range statement, checking if
+// its index or value variables are used unsafely inside goroutines or deferred
+// function literals.
+func checkRangeLoop(f *File, node ast.Node) {
+	n := node.(*ast.RangeStmt)
+	key, _ := n.Key.(*ast.Ident)
+	val, _ := n.Value.(*ast.Ident)
+	if key == nil && val == nil {
+		return
+	}
+	sl := n.Body.List
+	if len(sl) == 0 {
+		return
+	}
+	var last *ast.CallExpr
+	switch s := sl[len(sl)-1].(type) {
+	case *ast.GoStmt:
+		last = s.Call
+	case *ast.DeferStmt:
+		last = s.Call
+	default:
+		return
+	}
+	lit, ok := last.Fun.(*ast.FuncLit)
+	if !ok {
+		return
+	}
+	ast.Inspect(lit.Body, func(n ast.Node) bool {
+		id, ok := n.(*ast.Ident)
+		if !ok || id.Obj == nil {
+			return true
+		}
+		if f.pkg.types[id].Type == nil {
+			// Not referring to a variable
+			return true
+		}
+		if key != nil && id.Obj == key.Obj || val != nil && id.Obj == val.Obj {
+			f.Bad(id.Pos(), "range variable", id.Name, "captured by func literal")
+		}
+		return true
+	})
+}