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/alecthomas/gometalinter/issue.go

@@ -0,0 +1,114 @@
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"sort"
+	"strings"
+	"text/template"
+)
+
+// DefaultIssueFormat used to print an issue
+const DefaultIssueFormat = "{{.Path}}:{{.Line}}:{{if .Col}}{{.Col}}{{end}}:{{.Severity}}: {{.Message}} ({{.Linter}})"
+
+// Severity of linter message
+type Severity string
+
+// Linter message severity levels.
+const (
+	Error   Severity = "error"
+	Warning Severity = "warning"
+)
+
+type Issue struct {
+	Linter     string   `json:"linter"`
+	Severity   Severity `json:"severity"`
+	Path       string   `json:"path"`
+	Line       int      `json:"line"`
+	Col        int      `json:"col"`
+	Message    string   `json:"message"`
+	formatTmpl *template.Template
+}
+
+// NewIssue returns a new issue. Returns an error if formatTmpl is not a valid
+// template for an Issue.
+func NewIssue(linter string, formatTmpl *template.Template) (*Issue, error) {
+	issue := &Issue{
+		Line:       1,
+		Severity:   Warning,
+		Linter:     linter,
+		formatTmpl: formatTmpl,
+	}
+	err := formatTmpl.Execute(ioutil.Discard, issue)
+	return issue, err
+}
+
+func (i *Issue) String() string {
+	if i.formatTmpl == nil {
+		col := ""
+		if i.Col != 0 {
+			col = fmt.Sprintf("%d", i.Col)
+		}
+		return fmt.Sprintf("%s:%d:%s:%s: %s (%s)", strings.TrimSpace(i.Path), i.Line, col, i.Severity, strings.TrimSpace(i.Message), i.Linter)
+	}
+	buf := new(bytes.Buffer)
+	_ = i.formatTmpl.Execute(buf, i)
+	return buf.String()
+}
+
+type sortedIssues struct {
+	issues []*Issue
+	order  []string
+}
+
+func (s *sortedIssues) Len() int      { return len(s.issues) }
+func (s *sortedIssues) Swap(i, j int) { s.issues[i], s.issues[j] = s.issues[j], s.issues[i] }
+
+func (s *sortedIssues) Less(i, j int) bool {
+	l, r := s.issues[i], s.issues[j]
+	return CompareIssue(*l, *r, s.order)
+}
+
+// CompareIssue two Issues and return true if left should sort before right
+// nolint: gocyclo
+func CompareIssue(l, r Issue, order []string) bool {
+	for _, key := range order {
+		switch {
+		case key == "path" && l.Path != r.Path:
+			return l.Path < r.Path
+		case key == "line" && l.Line != r.Line:
+			return l.Line < r.Line
+		case key == "column" && l.Col != r.Col:
+			return l.Col < r.Col
+		case key == "severity" && l.Severity != r.Severity:
+			return l.Severity < r.Severity
+		case key == "message" && l.Message != r.Message:
+			return l.Message < r.Message
+		case key == "linter" && l.Linter != r.Linter:
+			return l.Linter < r.Linter
+		}
+	}
+	return true
+}
+
+// SortIssueChan reads issues from one channel, sorts them, and returns them to another
+// channel
+func SortIssueChan(issues chan *Issue, order []string) chan *Issue {
+	out := make(chan *Issue, 1000000)
+	sorted := &sortedIssues{
+		issues: []*Issue{},
+		order:  order,
+	}
+	go func() {
+		for issue := range issues {
+			sorted.issues = append(sorted.issues, issue)
+		}
+		sort.Sort(sorted)
+		for _, issue := range sorted.issues {
+			out <- issue
+		}
+		close(out)
+	}()
+	return out
+}