Compile with g++-4.7

g++-4.7 lacks thread_local, which can be workaround by
--disable-threads.  What left remaining is std::map::emplace, which is
what this change deals with.  First check availability of
std::map::emplace, if there is none, use std::map::insert.

configure.ac

@@ -197,6 +197,22 @@ std::vector<std::future<int>> v;
     [have_std_future=no
      AC_MSG_RESULT([no])])
 
+# Check that std::map::emplace is available for g++-4.7.
+AC_MSG_CHECKING([whether std::map::emplace is available])
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
+[[
+#include <map>
+]],
+[[
+std::map<int, int>().emplace(1, 2);
+]])],
+    [AC_DEFINE([HAVE_STD_MAP_EMPLACE], [1],
+               [Define to 1 if you have the `std::map::emplace`.])
+     have_std_map_emplace=yes
+     AC_MSG_RESULT([yes])],
+    [have_std_map_emplace=no
+     AC_MSG_RESULT([no])])
+
 AC_LANG_POP()
 
 # Checks for libraries.

src/shrpx_downstream_queue.cc

@@ -55,7 +55,12 @@ DownstreamQueue::HostEntry &
 DownstreamQueue::find_host_entry(const std::string &host) {
   auto itr = host_entries_.find(host);
   if (itr == std::end(host_entries_)) {
+#ifdef HAVE_STD_MAP_EMPLACE
     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()});
+#endif // !HAVE_STD_MAP_EMPLACE
   }
   return (*itr).second;
 }