Allow forcing of bundled rapidjson

[andresailer]

Dear Developers,

Would it be possible to add a cmake option to let us force the use of the bundled or external rapid json?

davix/CMakeLists.txt

Line 100 in f41e619

find_package(RapidJSON QUIET)

It would be unfortunate if we accidentally pick up one or the other depending on the build order of our software stacks (In my case building LCG software stacks).

Thanks!