Locating libconfig from CMake-based projects

Locating libconfig from CMake-based projects

Configuration files for CMake are provided to simplify
locating libconfig from within project that use CMake
as their build system.
Separate configuration files are provided for C and C++
builds.

In a CMakeLists.txt file, use the following commands to
locate libconfig:

- To locate the library, use either
    find_package(libconfig)
  or
    find_package(libconfig++)

- The include directories are available as variables
    LIBCONFIG_INCLUDE_DIRS
  and
    LIBCONFIG++_INCLUDE_DIRS
  respectively, and can be used like this:
    include_directories(${LIBCONFIG++_INCLUDE_DIRS})

- Link to libconfig using variables
    LIBCONFIG_LIBRARIES
  and
    LIBCONFIG++_LIBRARIES
  respectively, by writing:
    target_link_libraries(${PROJECT_NAME}
      ${LIBCONFIG++_LIBRARIES}
    )