Merge #2695: [CLI][Build] Guard libevent error messages

3570941 [CLI] Guard libevent error messages (Fuzzbawls)

Pull request description:

  The error message enum `evhttp_request_error` was introduced in libevent
  2.1.3, which we do not necessarily require, and is not available to some
  older but still supported linux distros (namely Ubuntu Xenial 16.04
  LTS).

  Add a version guard to maintain compatibility with the 2.0.x branch of
  libevent.

ACKs for top commit:
  random-zebra:
    utACK 3570941
  furszy:
    utACK 3570941

Tree-SHA512: c8914bb5e049eca8006c56eebcadb554594becd54c69a16becfb70505958f2ae998cc8a24b57340bdf8ea97f2c6637704512b9e796d098c9bb817b63fdc1b44c

src/pivx-cli.cpp

@@ -140,6 +140,7 @@ struct HTTPReply
 const char *http_errorstring(int code)
 {
     switch(code) {
+#if LIBEVENT_VERSION_NUMBER >= 0x02010300
         case EVREQ_HTTP_TIMEOUT:
             return "timeout reached";
         case EVREQ_HTTP_EOF:
@@ -152,6 +153,7 @@ const char *http_errorstring(int code)
             return "request was canceled";
         case EVREQ_HTTP_DATA_TOO_LONG:
             return "response body is larger than allowed";
+#endif
         default:
             return "unknown";
     }
@@ -182,11 +184,13 @@ static void http_request_done(struct evhttp_request *req, void *ctx)
     }
 }
 
+#if LIBEVENT_VERSION_NUMBER >= 0x02010300
 static void http_error_cb(enum evhttp_request_error err, void *ctx)
 {
     HTTPReply *reply = static_cast<HTTPReply*>(ctx);
     reply->error = err;
 }
+#endif
 
 UniValue CallRPC(const std::string& strMethod, const UniValue& params)
 {
@@ -204,7 +208,9 @@ UniValue CallRPC(const std::string& strMethod, const UniValue& params)
     raii_evhttp_request req = obtain_evhttp_request(http_request_done, (void*)&response);
     if (req == NULL)
         throw std::runtime_error("create http request failed");
+#if LIBEVENT_VERSION_NUMBER >= 0x02010300
     evhttp_request_set_error_cb(req.get(), http_error_cb);
+#endif
 
     // Get credentials
     std::string strRPCUserColonPass;