Avoid compilation warning under OS X 10.11

NSWindow.allowsAutomaticWindowTabbing property
is available since SDK 10.12.

src/osx/cocoa/utils.mm

@@ -478,10 +478,12 @@ void wxApp::DoCleanUp()
 void wxApp::OSXEnableAutomaticTabbing(bool enable)
 {
     // Automatic tabbing was first introduced in 10.12
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_12
     if ( WX_IS_MACOS_AVAILABLE(10, 12) )
     {
         [NSWindow setAllowsAutomaticWindowTabbing:enable];
     }
+#endif // macOS 10.12+
 }
 
 extern // used from src/osx/core/display.cpp