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/nilfunc.go

@@ -0,0 +1,67 @@
+// Copyright 2013 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 for useless function comparisons.
+A useless comparison is one like f == nil as opposed to f() == nil.
+*/
+
+package main
+
+import (
+	"go/ast"
+	"go/token"
+	"go/types"
+)
+
+func init() {
+	register("nilfunc",
+		"check for comparisons between functions and nil",
+		checkNilFuncComparison,
+		binaryExpr)
+}
+
+func checkNilFuncComparison(f *File, node ast.Node) {
+	e := node.(*ast.BinaryExpr)
+
+	// Only want == or != comparisons.
+	if e.Op != token.EQL && e.Op != token.NEQ {
+		return
+	}
+
+	// Only want comparisons with a nil identifier on one side.
+	var e2 ast.Expr
+	switch {
+	case f.isNil(e.X):
+		e2 = e.Y
+	case f.isNil(e.Y):
+		e2 = e.X
+	default:
+		return
+	}
+
+	// Only want identifiers or selector expressions.
+	var obj types.Object
+	switch v := e2.(type) {
+	case *ast.Ident:
+		obj = f.pkg.uses[v]
+	case *ast.SelectorExpr:
+		obj = f.pkg.uses[v.Sel]
+	default:
+		return
+	}
+
+	// Only want functions.
+	if _, ok := obj.(*types.Func); !ok {
+		return
+	}
+
+	f.Badf(e.Pos(), "comparison of function %v %v nil is always %v", obj.Name(), e.Op, e.Op == token.NEQ)
+}
+
+// isNil reports whether the provided expression is the built-in nil
+// identifier.
+func (f *File) isNil(e ast.Expr) bool {
+	return f.pkg.types[e].Type == types.Typ[types.UntypedNil]
+}