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/shift.go

@@ -0,0 +1,82 @@
+// Copyright 2014 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 suspicious shifts.
+*/
+
+package main
+
+import (
+	"go/ast"
+	"go/constant"
+	"go/token"
+	"go/types"
+)
+
+func init() {
+	register("shift",
+		"check for useless shifts",
+		checkShift,
+		binaryExpr, assignStmt)
+}
+
+func checkShift(f *File, node ast.Node) {
+	switch node := node.(type) {
+	case *ast.BinaryExpr:
+		if node.Op == token.SHL || node.Op == token.SHR {
+			checkLongShift(f, node, node.X, node.Y)
+		}
+	case *ast.AssignStmt:
+		if len(node.Lhs) != 1 || len(node.Rhs) != 1 {
+			return
+		}
+		if node.Tok == token.SHL_ASSIGN || node.Tok == token.SHR_ASSIGN {
+			checkLongShift(f, node, node.Lhs[0], node.Rhs[0])
+		}
+	}
+}
+
+// checkLongShift checks if shift or shift-assign operations shift by more than
+// the length of the underlying variable.
+func checkLongShift(f *File, node ast.Node, x, y ast.Expr) {
+	v := f.pkg.types[y].Value
+	if v == nil {
+		return
+	}
+	amt, ok := constant.Int64Val(v)
+	if !ok {
+		return
+	}
+	t := f.pkg.types[x].Type
+	if t == nil {
+		return
+	}
+	b, ok := t.Underlying().(*types.Basic)
+	if !ok {
+		return
+	}
+	var size int64
+	var msg string
+	switch b.Kind() {
+	case types.Uint8, types.Int8:
+		size = 8
+	case types.Uint16, types.Int16:
+		size = 16
+	case types.Uint32, types.Int32:
+		size = 32
+	case types.Uint64, types.Int64:
+		size = 64
+	case types.Int, types.Uint, types.Uintptr:
+		// These types may be as small as 32 bits, but no smaller.
+		size = 32
+		msg = "might be "
+	default:
+		return
+	}
+	if amt >= size {
+		ident := f.gofmt(x)
+		f.Badf(node.Pos(), "%s %stoo small for shift of %d", ident, msg, amt)
+	}
+}