Fix native OSX builds in test_sockets.py. For OSX, when our own-compiled fastcomp Clang is invoked to build a native executable, we must pass a command line parameter to it to tell which OSX SDK we are targeting.
Fix bad signature of emscripten_set_socket_error_callback() in emscripten.h
Silence a lot of the absolute path include warnings by always accepting any path that has the directory emscripten in it.