Make logging.c more readable by splitting up logging not implemented code path.
Make logging.c more readable by splitting up logging not implemented code path.

* logging.c
  (serf__log_init, serf__log_nopref, serf__log,
   serf_logging_create_stream_output, serf_logging_add_output): Define empty
  public functions when logging is not compiled-in.

blueszhangsh/serf,Manouchehri/serf,Manouchehri/serf,muthusuba/serf,TommyU/serf,idomon/serf,muthusuba/serf,krime/serf,blueszhangsh/serf,muthusuba/serf,wzugang/serf,justinweb/serf,liuzongquan/serf,idomon/serf,liuzongquan/serf,muthusuba/serf,TommyU/serf,liuzongquan/serf,wzugang/serf,blueszhangsh/serf,TommyU/serf,wzugang/serf,TommyU/serf,idomon/serf,wzugang/serf,krime/serf,liuzongquan/serf,sworda/serf,sworda/serf,blueszhangsh/serf,sworda/serf,krime/serf,Manouchehri/serf,justinweb/serf,justinweb/serf,krime/serf,sworda/serf,idomon/serf,Manouchehri/serf,justinweb/serf