Library names:
libcJSON -> libcjson
libcJSON_utils -> libcjson_utils

We configure and install a pkg-config file so that our compilation and
linking flags can be more easily found using pkg-config.

This is also what other projects are doing.

Turned off by default.

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.

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

The prior approach was exporting paths from the build environment.

This is for compatibility reasons with old CMake 2.8 versions.

And remove special handling of SunOs