message: elle.reactor: avoid warnings from clang about warnings from gcc
diff: # include <winsock2.h>
  #endif
  -#if defined __GNUC__
  +#if defined __GNUC__ && ! defined __clang__
  # pragma GCC diagnostic push
  // /usr/include/sys/poll.h:1:2: warning: #warning redirecting incorrect #include <sys/poll.h> to <poll.h> [-Wcpp]
  #include <boost/asio.hpp>
  -#if defined __GCC__
  +#if defined __GCC__ && ! defined __clang__
  # pragma GCC diagnostic pop
  #endif
  