Add pthread support for Emscripten build.

[aleph1]

I opened a pull request [#841] before seeing the note regarding the preference for issues.

The current CMakeLists.txt doesn’t include the required flags for Emscripten’s pthread support, which causes the built .wasm file to fail the MultithreadingTest when run in a browser (tested in Chrome, Safari, and Firefox for MacOS). Adding these flags results in all tests passing when running in a browser environment.