Problem building ofxPd using Emscripten

I tried building ofxPd/pdExample project using Emscripten following this instruction:

After running emmake make, it fails with the following error when it tries to build c files in libpd folder.

error: invalid argument '-std=c++14' not allowed with 'C'
shared:ERROR: compiler frontend failed to generate LLVM bitcode, halting

What should I do to fix this error? I would appreciate any advice.

P.S: I used openFrameworks-master branch since other versions have problem building Emscripten.