Configure: fixed msghdr.msg_control test on 64bit platforms.

Broken by r4560.

auto/unix

diff --git a/auto/unix b/auto/unix
index a3b95f9..783219e 100755
--- a/auto/unix
+++ b/auto/unix
@@ -703,8 +703,8 @@
                   #include <stdio.h>"
 ngx_feature_path=
 ngx_feature_libs=
-ngx_feature_test="struct msghdr  msg; msg.msg_control = NULL;
-                  printf(\"%d\", (int) msg.msg_control)"
+ngx_feature_test="struct msghdr  msg;
+                  printf(\"%d\", (int) sizeof(msg.msg_control))"
 . auto/feature