Make Celix consumable via FetchContent/add_subdirectory()

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

```CMAKE
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.