Allow configuring the update notifier from the outside

To test the update notifier:

    rm -f bin/gh; GH_VERSION=v0.2.3 LDFLAGS='-X main.updaterEnabled=github/homebrew-gh' make

.goreleaser.yml

@@ -9,7 +9,7 @@ builds:
       - -s -w -X github.com/github/gh-cli/command.Version={{.Version}} -X github.com/github/gh-cli/command.BuildDate={{.Date}}
       - -X github.com/github/gh-cli/context.oauthClientID={{.Env.GH_OAUTH_CLIENT_ID}}
       - -X github.com/github/gh-cli/context.oauthClientSecret={{.Env.GH_OAUTH_CLIENT_SECRET}}
-      - -X github.com/github/gh-cli.updaterEnabled=yes
+      - -X main.updaterEnabled=github/homebrew-gh
     goos:
       - linux
       - darwin

Makefile

@@ -1,7 +1,7 @@
 BUILD_FILES = $(shell go list -f '{{range .GoFiles}}{{$$.Dir}}/{{.}}\
 {{end}}' ./...)
 
-GH_VERSION = $(shell git describe --tags 2>/dev/null || git rev-parse --short HEAD)
+GH_VERSION ?= $(shell git describe --tags 2>/dev/null || git rev-parse --short HEAD)
 LDFLAGS := -X github.com/github/gh-cli/command.Version=$(GH_VERSION) $(LDFLAGS)
 LDFLAGS := -X github.com/github/gh-cli/command.BuildDate=$(shell date +%Y-%m-%d) $(LDFLAGS)
 ifdef GH_OAUTH_CLIENT_SECRET

main.go

@@ -12,7 +12,7 @@ import (
 	"github.com/mgutz/ansi"
 )
 
-var updaterEnabled = "no"
+var updaterEnabled = ""
 
 func main() {
 	currentVersion := command.Version
@@ -42,7 +42,7 @@ Release notes: %s`, currentVersion, newRelease.Version, newRelease.URL)
 
 func shouldCheckForUpdate() bool {
 	errFd := os.Stderr.Fd()
-	return updaterEnabled == "yes" && (isatty.IsTerminal(errFd) || isatty.IsCygwinTerminal(errFd))
+	return updaterEnabled != "" && (isatty.IsTerminal(errFd) || isatty.IsCygwinTerminal(errFd))
 }
 
 func checkForUpdate(currentVersion string) (*update.ReleaseInfo, error) {
@@ -55,5 +55,6 @@ func checkForUpdate(currentVersion string) (*update.ReleaseInfo, error) {
 		return nil, err
 	}
 
-	return update.CheckForUpdate(client, "github/homebrew-gh", currentVersion)
+	repo := updaterEnabled
+	return update.CheckForUpdate(client, repo, currentVersion)
 }