Made global thread pool shutdown not attempted unless the thread pool was

already enabled.  This also makes the global destruction order a little more
deterministic.

dlib/threads/threads_kernel_shared.cpp

@@ -39,15 +39,6 @@ namespace dlib
 
         bool thread_pool_has_been_destroyed = false;
 
-// ----------------------------------------------------------------------------------------
-
-        threader& thread_pool (
-        ) 
-        {
-            static threader* thread_pool = new threader;
-            return *thread_pool;
-        }
-
 // ----------------------------------------------------------------------------------------
 
         struct threader_destruct_helper
@@ -59,7 +50,16 @@ namespace dlib
                 thread_pool().destruct_if_ready();
             }
         };
-        static threader_destruct_helper a;
+
+// ----------------------------------------------------------------------------------------
+
+        threader& thread_pool (
+        ) 
+        {
+            static threader* thread_pool = new threader;
+            static threader_destruct_helper a;
+            return *thread_pool;
+        }
 
 // ----------------------------------------------------------------------------------------