Make Celix consumable via FetchContent/add_subdirectory()

[PengZheng]

Currently Celix can not be added as a sub-project via add_subdirectory. For example, use of CMAKE_SOURCE_DIR will cause build error.

function(celix_deprecated_utils_headers)
    list(GET ARGN 0 TARGET_NAME)
    target_include_directories(${TARGET_NAME} PRIVATE ${CMAKE_SOURCE_DIR}/libs/utils/include_deprecated)
    set_property(TARGET celix-deprecated APPEND PROPERTY "UTIL_TARGETS" "${TARGET_NAME}")
endfunction()

After finish this, we could add sub-project build to our CI.