Allow JsonCpp to be VTK or system supplied

Added a simple FindJsonCpp.cmake to find a system JsonCpp, added the VTK
forwarding header along with a check to confirm that it was the JsonCpp
header and not the C JSON library with an identically named header. This
addresses bug #14155.

Change-Id: Ic9287af538a552510162c77faa6e8f15161aea5f
Merge topic 'jsoncpp-system-option' into master

6c18d39e Allow JsonCpp to be VTK or system supplied