def update_extend(dst, src):
for k, v in src.items():<EOL><INDENT>existing = dst.setdefault(k, [])<EOL>for x in v:<EOL><INDENT>if x not in existing:<EOL><INDENT>existing.append(x)<EOL><DEDENT><DEDENT><DEDENT>
Update the `dst` with the `src`, extending values where lists.

    Primiarily useful for integrating results from `get_library_config`.