Android JNI doesn't find C++ standard library header files

I am following the android-studio-jni guide and trying to compile C++ native code with gradle. Everything works great: I can load the JNI function, write C++ classes, compile, run and debug. And I don't need to write and maintain the Application.mk and Android.mk makefiles; both of them seems to be handled by gradle implicitly. There is only one thing that I don't understand: How to include C++ header files from the standard libraries?

I think I must have missed something in the gradle script, but I cannot find a good reference about what to add here. Here is the script:

android.ndk {
    moduleName = "hello-android-jni"

    // I tried adding the following, but nothing happens
    stl = "stlport_static"
}

And screenshots showing that all the std headers are not visible.