Add a method to show if the c-ares library is used (#88)

Author: an-tao

trantor/net/Resolver.h

@@ -23,5 +23,6 @@ class Resolver
     virtual ~Resolver()
     {
     }
+    static bool isCAresUsed();
 };
 }  // namespace trantor

trantor/net/inner/AresResolver.cc

@@ -44,6 +44,11 @@ const char* getSocketType(int type)
 
 }  // namespace
 
+bool Resolver::isCAresUsed()
+{
+    return true;
+}
+
 AresResolver::LibraryInitializer::LibraryInitializer()
 {
     ares_library_init(ARES_LIB_INIT_ALL);

trantor/net/inner/NormalResolver.cc

@@ -17,7 +17,10 @@ std::shared_ptr<Resolver> Resolver::newResolver(trantor::EventLoop *loop,
 {
     return std::make_shared<NormalResolver>(timeout);
 }
-
+bool Resolver::isCAresUsed()
+{
+    return false;
+}
 void NormalResolver::resolve(const std::string &hostname,
                              const Callback &callback)
 {