Fix compile error with gcc-4.7

Use std::map::insert instead of std::map::emplace, since gcc-4.7 does
not support the latter.

src/HttpServer.cc

@@ -260,7 +260,12 @@ public:
     return &ent;
   }
   FileEntry *cache_fd(const std::string &path, const FileEntry &ent) {
+#ifdef HAVE_STD_MAP_EMPLACE
     auto rv = fd_cache_.emplace(path, ent);
+#else  // !HAVE_STD_MAP_EMPLACE
+    // for gcc-4.7
+    auto rv = fd_cache_.insert(std::make_pair(path, ent));
+#endif // !HAVE_STD_MAP_EMPLACE
     return &(*rv.first).second;
   }
   void release_fd(const std::string &path) {

src/shrpx_downstream_queue.cc

@@ -64,7 +64,8 @@ DownstreamQueue::find_host_entry(const std::string &host) {
     std::tie(itr, std::ignore) = host_entries_.emplace(host, HostEntry());
 #else  // !HAVE_STD_MAP_EMPLACE
     // for g++-4.7
-    std::tie(itr, std::ignore) = host_entries_.insert({host, HostEntry()});
+    std::tie(itr, std::ignore) =
+        host_entries_.insert(std::make_pair(host, HostEntry()));
 #endif // !HAVE_STD_MAP_EMPLACE
   }
   return (*itr).second;