build: Fix build when std::to_string is not available

Change-Id: If34fcebe17bd7e3de23655680957fa6e0df27676
Refs: #4393
diff --git a/tools/ndn-autoconfig/ndn-fch-discovery.cpp b/tools/ndn-autoconfig/ndn-fch-discovery.cpp
index 395d9f9..5365496 100644
--- a/tools/ndn-autoconfig/ndn-fch-discovery.cpp
+++ b/tools/ndn-autoconfig/ndn-fch-discovery.cpp
@@ -190,7 +190,7 @@
     }
     if (statusCode != 200) {
       boost::trim(statusMessage);
-      BOOST_THROW_EXCEPTION(HttpException("HTTP request failed: " + std::to_string(statusCode) + " " + statusMessage));
+      BOOST_THROW_EXCEPTION(HttpException("HTTP request failed: " + to_string(statusCode) + " " + statusMessage));
     }
     std::string header;
     while (std::getline(requestStream, header) && header != "\r")