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...
> C preprocessor. ie -DBOOST_SPIRIT_THREADSAFE needs to be passed to the C++ compiler.

`cmake -DCMAKE_C_FLAGS=-DBOOST_SPIRIT_THREADSAFE -DCMAKE_CXX_FLAGS=-DBOOST_SPIRIT_THREADSAFE` should be enough then