emscripten C++11 with boost support issue

I have an issue when building my C++ project that uses boost in Emscripten as shown on the screenshot, it says that '_Atomic' is a C11 extension however even if i add -std=c++11 or even -std=c11 i am still getting the error, the _Atomic use is defined from boost.

any idea on how to work around on this? reading around it does say that C++11 is already supported in Emscripten.

my setup. compiler: emscripten/em++ Clang 12.0.8