How do we log C stuff with stack trace from Emscripten?..
Like this!
At top of file:

    #ifdef __EMSCRIPTEN__
    #include "../emscripten.h"
    #endif

...and then, later:

    #ifdef __EMSCRIPTEN__
    emscripten_log(EM_LOG_CONSOLE | EM_LOG_C_STACK, "WTF: %s\n", some_string);
    #endif

See also:
* emscripten_log: https://emscripten.org/docs/api_reference/emscripten.h.html#id8
* EM_LOG_CONSOLE, EM_LOG_C_STACK, etc: https://emscripten.org/docs/api_reference/emscripten.h.html#logging-utilities