build fail on arm32

[welees]

When I build with arm-linux-gnueabi-gcc, the compiler report

http/server/HttpHandler.cpp:241:65: error: invalid use of incomplete type ‘class std::future<void>’
         hv::async(std::bind(handler->async_handler, req, writer));
                                                                 ^
In file included from cpputil/hthreadpool.h:16:0,
                 from cpputil/hasync.h:5,
                 from http/server/HttpHandler.cpp:8:
/usr/arm-linux-gnueabi/include/c++/5/future:115:11: note: declaration of ‘class std::future<void>’
     class future;
           ^
In file included from http/server/HttpHandler.cpp:8:0:
cpputil/hasync.h: In instantiation of ‘std::future<decltype (fn(hv::async::args ...))> hv::async(Fn&&, Args&& ...) [with Fn = std::_Bind<std::function<void(const std::shared_ptr<HttpRequest>&, const std::shared_ptr<hv::HttpResponseWriter>&)>(std::shared_ptr<HttpRequest>, std::shared_ptr<hv::HttpResponseWriter>)>; Args = {}; decltype (fn(hv::async::args ...)) = void]’:
http/server/HttpHandler.cpp:241:65:   required from here
cpputil/hasync.h:25:6: error: return type ‘class std::future<void>’ is incomplete
 auto async(Fn&& fn, Args&&... args) -> std::future<decltype(fn(args...))> {
      ^
cpputil/hasync.h:26:98: error: invalid use of incomplete type ‘class std::future<void>’
     return GlobalThreadPool::instance()->commit(std::forward<Fn>(fn), std::forward<Args>(args)...);

```                                                                                                  ^