A simple implementation of #1491. (Annotating functions to be exported in the C/C++ source)

* All exported functions have to be marked with EM_EXPORT which is defined in [system/include/emscripten/emscripten.h](https://github.com/ngld/emscripten/blob/auto_export/system/include/emscripten/emscripten.h#L38).
* You have to pass -fvisibility=hidden to the C compiler.
* You have to pass -s AUTO_EXPORT=1 -s LINKABLE=1 to emscripten when converting the bitcode to JS.