proc/native,Makefile: allow compiling on macOS without native backend

On macOS 10.14 Apple changed the command line tools so that system
headers now need to be manually installed.

Instead of adding one extra install step to the install procedure add a
build tag to allow compilation of delve without the native backend on
macOS. By default (i.e. when using `go get`) this is how delve will be
compiled on macOS, the make script is changed to enable compiling the
native backend if the required dependencies have been installed.

Insure that both configuration still build correctly on Travis CI and
change the documentation to describe how to compile the native backend
and that it isn't normally needed.

Fixes #1359

scripts/make.go

@@ -34,9 +34,9 @@ func NewMakeCommands() *cobra.Command {
 		Use:   "build",
 		Short: "Build delve",
 		Run: func(cmd *cobra.Command, args []string) {
-			checkCertCmd(nil, nil)
-			execute("go", "build", buildFlags(), DelveMainPackagePath)
-			if runtime.GOOS == "darwin" && os.Getenv("CERT") != "" {
+			tagFlag := prepareMacnative()
+			execute("go", "build", tagFlag, buildFlags(), DelveMainPackagePath)
+			if runtime.GOOS == "darwin" && os.Getenv("CERT") != "" && canMacnative() {
 				codesign("./dlv")
 			}
 		},
@@ -46,9 +46,9 @@ func NewMakeCommands() *cobra.Command {
 		Use:   "install",
 		Short: "Installs delve",
 		Run: func(cmd *cobra.Command, args []string) {
-			checkCertCmd(nil, nil)
-			execute("go", "install", buildFlags(), DelveMainPackagePath)
-			if runtime.GOOS == "darwin" {
+			tagFlag := prepareMacnative()
+			execute("go", "install", tagFlag, buildFlags(), DelveMainPackagePath)
+			if runtime.GOOS == "darwin" && os.Getenv("CERT") != "" && canMacnative() {
 				codesign(installedExecutablePath())
 			}
 		},
@@ -183,6 +183,33 @@ func installedExecutablePath() string {
 	return filepath.Join(strings.TrimSpace(gopath[0]), "bin", "dlv")
 }
 
+// canMacnative returns true if we can build the native backend for macOS,
+// i.e. cgo enabled and the legacy SDK headers:
+// https://forums.developer.apple.com/thread/104296
+func canMacnative() bool {
+	if runtime.GOOS != "darwin" {
+		return false
+	}
+	if strings.TrimSpace(getoutput("go", "env", "CGO_ENABLED")) != "1" {
+		return false
+	}
+	_, err := os.Stat("/usr/include/sys/types.h")
+	if err != nil {
+		return false
+	}
+	return true
+}
+
+// prepareMacnative checks if we can build the native backend for macOS and
+// if we can checks the certificate and then returns the -tags flag.
+func prepareMacnative() string {
+	if !canMacnative() {
+		return ""
+	}
+	checkCertCmd(nil, nil)
+	return "-tags=macnative"
+}
+
 func buildFlags() []string {
 	buildSHA, err := exec.Command("git", "rev-parse", "HEAD").CombinedOutput()
 	if err != nil {
@@ -214,6 +241,13 @@ func testCmd(cmd *cobra.Command, args []string) {
 	checkCertCmd(nil, nil)
 
 	if os.Getenv("TRAVIS") == "true" && runtime.GOOS == "darwin" {
+		fmt.Println("Building with native backend")
+		execute("go", "build", "-tags=macnative", buildFlags(), DelveMainPackagePath)
+
+		fmt.Println("\nBuilding without native backend")
+		execute("go", "build", buildFlags(), DelveMainPackagePath)
+
+		fmt.Println("\nTesting")
 		os.Setenv("PROCTEST", "lldb")
 		executeq("sudo", "-E", "go", "test", testFlags(), allPackages())
 		return