The build system (at least autotools-based) should detect and configure
emscripten correctly.

As of now, gctest almost passes, except for the tests that involve a_get().

No thread support for now. No idea how to stop other threads (perhaps we need
support from JS side).

How to build:

    # source EMSDK first
    emconfigure ./configure
    emmake make gctest.html
    # point your browser at .libs/gctest.html