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