This is still in a very early stage.
Currently you have to build V8 yourself and copy the libraries and headers manually.
See V8's download and build instructions.
After building V8, run the following commands from this repositories root directory:
$ cp -r <path-to-google-v8>/out/native/obj.target/tools/gyp/ lib
$ cp -r <path-to-google-v8>/include/ .
$ cabal configure --enable-tests && cabal build && ./dist/build/spec/spec