Fix CMake subproject behaviour.

If you add jsoncpp to another project with CMake, as a subproject, than ${CMAKE_SOURCE_DIR}/include will be a root project include directory:

    project_a/
      CMakeLists.txt  # Contains: add_subdirectory(jsoncpp)
      include/  # <- ${CMAKE_SOURCE_DIR}/include
      jsoncpp/
        CMakeLists.txt
        include/  # <- ${jsoncpp_SOURCE_DIR}/include