Reachability class error
<p>Xcode 4.2, iOS 5.0
My project is ARC enabled, When add non-ARC Reachabilty class in my project, then I got following error:</p>

<pre><code>Undefined symbols for architecture i386:
  "_SCNetworkReachabilitySetCallback", referenced from:
      -[Reachability startNotifier] in Reachability.o
  "_SCNetworkReachabilityScheduleWithRunLoop", referenced from:
      -[Reachability startNotifier] in Reachability.o
  "_SCNetworkReachabilityUnscheduleFromRunLoop", referenced from:
      -[Reachability stopNotifier] in Reachability.o
  "_SCNetworkReachabilityCreateWithName", referenced from:
      +[Reachability reachabilityWithHostName:] in Reachability.o
  "_SCNetworkReachabilityCreateWithAddress", referenced from:
      +[Reachability reachabilityWithAddress:] in Reachability.o
  "_SCNetworkReachabilityGetFlags", referenced from:
      -[Reachability connectionRequired] in Reachability.o
      -[Reachability currentReachabilityStatus] in Reachability.o
ld: symbol(s) not found for architecture i386
clang: error: linker command failed with exit code 1 (use -v to see invocation)
</code></pre>
