Spurred by Kitware/CMake#144

Basically, jsoncpp is generating the config file in an "old" or less-preferred way. I tried patching the Find module slated for inclusion in CMake 3.2.0 to be able to bootstrap itself from the config file, but the (somewhat hasty) pull request was (quite reasonably) denied because of dependence on implementation-specific details of exported targets.

Not sure exactly what needs to be done to improve this, but I'll be starting a thread on the CMake mailing list to follow up on that patch.