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...
$(PKG)_UPDATE is a recursively expanded variable, which means when the main makefile references this $(PKG) might be something else (as we include all the individual makefiles before any operation is done).