code: protected function selectBuilder()
      {
          if (PHP_OS === 'Darwin') {
              if (exec('which growlnotify')) {
                  return new GrowlNotifyBuilder;
              }
              if (exec('which terminal-notifier')) {
                  return new TerminalNotifierBuilder;
              }
          }
          if (PHP_OS === 'Linux') {
              if (exec('which notify-send')) {
                  return new NotifySendBuilder;
              }
          }
          if (PHP_OS === 'WINNT') {
              if (exec('where growlnotify')) {
                  return new GrowlNotifyWindowsBuilder;
              }
          }
      }
docs: Chooses a Builder to use depending on the operating system and which
  program is installed.
  
  @codeCoverageIgnore
  @return \BryanCrowe\Growl\Builder\BuilderAbstract A suitable Builder for
  a notification program that was found on the system.