CMAKE_INSTALL_FULL_... needs to be used.

This can be disabled when the exporting of the targets causes problems.

The prior approach was exporting paths from the build environment.

Fix library and include paths in pkg-config, CMakeLists.txt and the
CMake config files.

These files will allow cmake based projects to find the
library using find_package(cJSON). If successful, they can
use CJSON_LIBRARIES and CJSON_INCLUDE_DIRS.

Other build systems can use a similar approach to pkg-config:
cmake --find-package -DNAME=cJSON -DCOMPILER_ID=GNU -DLANGUAGE=C  -DCMAKE_PREFIX_PATH=<path-to-non-sytem-installation-path> -DMODE=LINK

or with MODE COMPILE or EXISTS.