Use cppcheck_add_target_sources_to_unused_function_check.

This has the handy side effect of adding the target itself as a
dependency to the unused function check, which is useful since
some generators don't appear to support running the rules to
generate files which are out of the directory immediate directory
tree.
Merge pull request #30 from smspillaz/common-universal-cmake.item_29

Use cppcheck_add_target_sources_to_unused_function_check