C++17 cannot include <filesystem>

I'm using latest CLion version with LLVM on OSX. I'm trying to play with new features of C++17 and one of the features I see is filesystem header file.

I'm trying to include it like this 

#include<filesystem>

namespace std = std::filesystem;

But the compiler is complaining the "file is not found". In my CMakeLists.txt I have :

cmake_minimum_required(VERSION 3.9)
project(testrunner)

set(CMAKE_CXX_STANDARD 17)

add_executable(testrunner main.cpp)

And I did select C++17 project on the begging. What am I doing wrong? Is there any special flag or option to enable the syntax highlighting and compilation?

Thank you for your help.