Ensure compilation to Javascript with Emscripten

This small commit ensure that (the core of) OpenCV can compile to JavaScript with Emscripten.

Successfully tested with `core`, `imgproc`, `calib3d`, `feature2d`, `video`, `highgui`.

Note that one pending issue is still required in Emscripten: https://github.com/kripken/emscripten/issues/1860