runtime: skip TestCgoExternalThreadSIGPROF on OS X 10.6

The test program requires static constructor, which in turn needs
external linking to work, but external linking never works on 10.6.

This should fix the darwin-{386,amd64} builders.

Change-Id: I714fdd3e35f9a7e5f5659cf26367feec9412444f
Reviewed-on: https://go-review.googlesource.com/2235Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/crash_cgo_test.go

@@ -9,6 +9,7 @@ package runtime_test
 import (
 	"runtime"
 	"strings"
+	"syscall"
 	"testing"
 )
 
@@ -52,8 +53,20 @@ func TestCgoExternalThreadPanic(t *testing.T) {
 
 func TestCgoExternalThreadSIGPROF(t *testing.T) {
 	// issue 9456.
-	if runtime.GOOS == "plan9" || runtime.GOOS == "windows" {
+	switch runtime.GOOS {
+	case "plan9", "windows":
 		t.Skipf("no pthreads on %s", runtime.GOOS)
+	case "darwin":
+		// static constructor needs external linking, but we don't support
+		// external linking on OS X 10.6.
+		osver, err := syscall.Sysctl("kern.osrelease")
+		if err != nil {
+			t.Fatalf("Sysctl(kern.osrelease) failed: %v", err)
+		}
+		// OS X 10.6 == Darwin 10.x
+		if strings.HasPrefix(osver, "10.") {
+			t.Skipf("no external linking on OS X 10.6")
+		}
 	}
 	got := executeTest(t, cgoExternalThreadSIGPROFSource, nil)
 	want := "OK\n"