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>