And remove special handling of SunOs

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

This is for compatibility reasons with old CMake 2.8 versions.

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.

Turned off by default.

This is also what other projects are doing.

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

Library names:
libcJSON -> libcjson
libcJSON_utils -> libcjson_utils