Uninitialized CMake variable `JSONCPP_VERSION`

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Describe the bug
In configure_file("${PROJECT_SOURCE_DIR}/version.in" "${PROJECT_BINARY_DIR}/version" NEWLINE_STYLE UNIX), the variable JSONCPP_VERSION used in the template header is not set. This causes the version file in the binary directory to be empty.

CMake Warning (dev) at src/CDK/CORELIB/MAIN/3rdparty/jsoncpp/CMakeLists.txt:100 (configure_file):
  uninitialized variable 'JSONCPP_VERSION'

You probably meant to use the jsoncpp_VERSION variable in version.in.

To Reproduce
Steps to reproduce the behavior:

1. Configure project with CMake. Use --warn-uninitialized to see the warning.

Expected behavior
The correct variable is used and the project version is written to the version file.