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

Currently, the static and dynamic version of the libraries are always
built as add_library is called twice. Instead, this patch will use the
standard CMake variable BUILD_SHARED_LIBS to know if the static or the
dynamic version must be built.
Signed-off-by: NFabrice Fontaine <fabrice.fontaine@orange.com>

Signed-off-by: NAnton Sergeev <Anton.Sergeev@elecard.ru>

Signed-off-by: NAnton Sergeev <Anton.Sergeev@elecard.ru>