// linking is not required.
func externalLinkingReason(p *Package) (what string) {
	// Some targets must use external linking even inside GOROOT.
	if platform.MustLinkExternal(cfg.Goos, cfg.Goarch, false) {
		return cfg.Goos + "/" + cfg.Goarch
	}

	// Some build modes always require external linking.
	switch cfg.BuildBuildmode {
	case "c-shared", "plugin":