Support multithreading

[woodser]

Closes #96

This PR updates most of the configuration to support multithreading.

The build completes most of the way but fails with the following error during final linking:

[100%] Linking CXX executable monero_wallet_full.js
Error running link command: No such file or directory
make[2]: *** [monero_wallet_full.js] Error 2
make[1]: *** [CMakeFiles/monero_wallet_full.dir/all] Error 2
make: *** [all] Error 2
emmake: error: 'cmake --build . -j16' failed (returned 2)

Any help would be greatly appreciated. Starting points could be reading the latest documentation to support pthreads in emscripten, and playing with the shared options in build_openssl_emscripten.sh.

Before attempting to build, these folders should first be deleted so the full stack is built with pthreads enabled:

./build ./external/monero-cpp/external/boost-sdk ./external/monero-cpp/external/openssl-sdk