New macro ASIO_DISABLE_CONNECTEX to disable ConnectEx use.

asio/include/asio/detail/impl/win_iocp_socket_service_base.ipp

diff --git a/asio/include/asio/detail/impl/win_iocp_socket_service_base.ipp b/asio/include/asio/detail/impl/win_iocp_socket_service_base.ipp
index 6f7886e..df3975a 100644
--- a/asio/include/asio/detail/impl/win_iocp_socket_service_base.ipp
+++ b/asio/include/asio/detail/impl/win_iocp_socket_service_base.ipp
@@ -673,6 +673,11 @@
 win_iocp_socket_service_base::get_connect_ex(
     win_iocp_socket_service_base::base_implementation_type& impl, int type)
 {
+#if defined(ASIO_DISABLE_CONNECTEX)
+  (void)impl;
+  (void)type;
+  return 0;
+#else // defined(ASIO_DISABLE_CONNECTEX)
   if (type != ASIO_OS_DEF(SOCK_STREAM)
       && type != ASIO_OS_DEF(SOCK_SEQPACKET))
     return 0;
@@ -696,6 +701,7 @@
   }
 
   return reinterpret_cast<connect_ex_fn>(ptr == this ? 0 : ptr);
+#endif // defined(ASIO_DISABLE_CONNECTEX)
 }
 
 void* win_iocp_socket_service_base::interlocked_compare_exchange_pointer(