json_spirit package

json_spirit builds out of the box as a cmake project. The only gotcha is that the original source site protects the download URL with a login form, so I have had mirror the original package on my website.

Also not sure what the update target is supposed to do...
MXE does support threading (see w32threads and pthreads-win32 packages), and Boost package seems to enable it:

```
        threading=multi \
        link=$(if $(BUILD_STATIC),static,shared) \
        threadapi=win32 \
```

So should we define `BOOST_SPIRIT_THREADSAFE`?