shared_lib_deps.pl: added separate --debug option
shared_lib_deps.pl: added defined() method in Config
Cache: Break recursive dependencies issue; fixes #4

- Use a flag ("forced_deps") when recursing in order to prevent
  further recursion
shared_lib_deps.pl: change output log format
Cache: document 'recursion_allowed' flag; fixes #4

- Change 'recurse' parameter to 'recurse_file