build.go: Make `-ldflags` compatible to Go 1.5

This change uses the old syntax (-ldflags "-X foo bar") for Go <= 1.4
and the new syntax for (-ldflags "-X foo=bar") for Go 1.5 (without a
warning).

build.go

@@ -229,6 +229,27 @@ func gitVersion() string {
 	return version
 }
 
+// Constants represents a set constants set in the final binary to the given
+// value.
+type Constants map[string]string
+
+// LDFlags returns the string that can be passed to go build's `-ldflags`.
+func (cs Constants) LDFlags() string {
+	l := make([]string, 0, len(cs))
+
+	if strings.HasPrefix(runtime.Version(), "go1.5") {
+		for k, v := range cs {
+			l = append(l, fmt.Sprintf(`-X "%s=%s"`, k, v))
+		}
+	} else {
+		for k, v := range cs {
+			l = append(l, fmt.Sprintf(`-X %q %q`, k, v))
+		}
+	}
+
+	return strings.Join(l, " ")
+}
+
 func main() {
 	buildTags := []string{}
 
@@ -307,10 +328,13 @@ func main() {
 	}
 	version := getVersion()
 	compileTime := time.Now().Format(timeFormat)
-	ldflags := fmt.Sprintf("-s -X main.compiledAt %q", compileTime)
+	constants := Constants{`main.compiledAt`: compileTime}
 	if version != "" {
-		ldflags = fmt.Sprintf("%s -X main.version %q", ldflags, version)
+		constants["main.version"] = version
 	}
+	ldflags := "-s " + constants.LDFlags()
+	fmt.Printf("ldflags: %s\n", ldflags)
+
 	args := []string{
 		"-tags", strings.Join(buildTags, " "),
 		"-ldflags", ldflags,