Before emscripten v1.38.27 and C++17

This release requires emscripten v1.37.1.

To install emscritpen v1.37.1,

git clone https://github.com/emscripten-core/emsdk.git
cd emsdk
./emsdk install emscripten-1.37.1
./emsdk activate emscripten-1.37.1
./emsdk install clang-e1.37.1-64bit
./emsdk activate clang-e1.37.1-64bit

Then, once per terminal session before compiling for web,

source path/to/emsdk/emsdk_env.sh