Changed the windows signaler and mutex code to use the C++11 thread library instead of

the old win32 functions.  I did this to work around how windows unloads dlls.  In particular, during dll unload
windows will kill all threads, THEN it will destruct global objects.  So this leads to problems
where a global obejct that owns threads tries to tell them to shutdown and everything goes wrong.
The specific problem this code change fixes is when signaler::broadcast() is called
on a signaler that was being waited on by one of these abruptly killed threads.  In that case, the old
code would deadlock inside signaler::broadcast().  This new code doesn't seem to have that problem,
thereby mitigating the windows dll unload behavior in some situations.

dlib/threads/threads_kernel_1.h

@@ -12,6 +12,9 @@
 #include "../windows_magic.h"
 #include <windows.h>
 #include "../algs.h"
+#include <condition_variable>
+#include <mutex>
+#include <chrono>
 
 
 namespace dlib
@@ -43,21 +46,22 @@ namespace dlib
         mutex (
         ) 
         {
-            InitializeCriticalSection(&cs);
         }
 
         ~mutex (
-        ) { DeleteCriticalSection(&cs); }
+        ) {  }
 
         void lock (
-        ) const { EnterCriticalSection(&cs); }
+        ) const { cs.lock(); }
 
         void unlock (
-        ) const { LeaveCriticalSection(&cs); }
+        ) const { cs.unlock(); }
 
     private:
 
-        mutable CRITICAL_SECTION cs;
+        friend class signaler;
+
+        mutable std::mutex cs;
 
         // restricted functions
         mutex(mutex&);        // copy constructor
@@ -77,147 +81,40 @@ namespace dlib
         signaler (
             const mutex& associated_mutex
         ) :
-            hSemaphore(CreateSemaphore (NULL, 0, 100000000, NULL)),
-            waiters(0),
-            hCountSema(CreateSemaphore (NULL,0,100000000,NULL)),
             m(associated_mutex)
         {           
-            if (hSemaphore == NULL || hCountSema == NULL)
-            {
-                if (hSemaphore != NULL)
-                {
-                    CloseHandle(hSemaphore); 
-                }
-
-                if (hCountSema != NULL)
-                {
-                    CloseHandle(hCountSema); 
-                }
 
-                throw dlib::thread_error(ECREATE_SIGNALER,
-        "in function signaler::signaler() an error occurred making the signaler"
-                );        
-            }
         }
 
         ~signaler (
-        ) { CloseHandle(hSemaphore); CloseHandle(hCountSema);}
+        ) { }
 
         void wait (
         ) const
         { 
-            // get a lock on the mutex for the waiters variable
-            waiters_mutex.lock();
-            // mark that one more thread will be waiting on this signaler
-            ++waiters;
-            // release the mutex for waiters
-            waiters_mutex.unlock();
-
-            // release the associated mutex
-            m.unlock();
-
-            // wait for the semaphore to be signaled
-            WaitForSingleObject (hSemaphore,INFINITE);
-
-            // signal that we are awake
-            ReleaseSemaphore(hCountSema,(LONG)1,NULL);
-
-            // relock the associated mutex 
-            m.lock();
+            std::unique_lock<std::mutex> cs(m.cs, std::defer_lock);
+            cv.wait(cs);
         }
 
         bool wait_or_timeout (
             unsigned long milliseconds
         ) const
         { 
-            // get a lock on the mutex for the waiters variable
-            waiters_mutex.lock();
-            // mark that one more thread will be waiting on this signaler
-            ++waiters;
-            // release the mutex for waiters
-            waiters_mutex.unlock();
-
-            // release the associated mutex
-            m.unlock();
-
-            bool value;
-
-            // wait for the semaphore to be signaled
-            if ( WaitForSingleObject (hSemaphore, milliseconds ) == WAIT_TIMEOUT )
-            {
-                // in this case we should decrement waiters because we are returning
-                // due to a timeout rather than because someone called signal() or 
-                // broadcast().
-                value = false;
-
-                // signal that we are awake
-                ReleaseSemaphore(hCountSema,(LONG)1,NULL);
-
-                // get a lock on the mutex for the waiters variable
-                waiters_mutex.lock();
-                // mark that one less thread will be waiting on this signaler. 
-                if (waiters != 0)
-                    --waiters;
-                // release the mutex for waiters
-                waiters_mutex.unlock();
-            }
-            else 
-            {
-                value = true;
-
-                // signal that we are awake
-                ReleaseSemaphore(hCountSema,(LONG)1,NULL);
-            }
-
-
-            // relock the associated mutex 
-            m.lock();
-
-            return value;
+            std::unique_lock<std::mutex> cs(m.cs, std::defer_lock);
+            auto status = cv.wait_until(cs, std::chrono::system_clock::now() + std::chrono::milliseconds(milliseconds));
+            return status == std::cv_status::no_timeout;
         }
 
         void signal (
         ) const 
         { 
-            // get a lock on the mutex for the waiters variable
-            waiters_mutex.lock();
-            
-            if (waiters > 0)
-            {
-                --waiters;
-                // make the semaphore release one waiting thread
-                ReleaseSemaphore(hSemaphore,1,NULL);
-
-                // wait for signaled thread to wake up
-                WaitForSingleObject(hCountSema,INFINITE);               
-            }
-
-            // release the mutex for waiters
-            waiters_mutex.unlock();
+            cv.notify_one();
         }
 
         void broadcast (
         ) const 
         { 
-            // get a lock on the mutex for the waiters variable
-            waiters_mutex.lock();
-            
-            if (waiters > 0)
-            {   
-                // make the semaphore release all the waiting threads
-                ReleaseSemaphore(hSemaphore,(LONG)waiters,NULL);
-
-                // wait for count to be zero
-                for (unsigned long i = 0; i < waiters; ++i)
-                {
-                    WaitForSingleObject(hCountSema,INFINITE);
-                }
-
-                waiters = 0;
-            }
-
-            // release the mutex for waiters
-            waiters_mutex.unlock();
+            cv.notify_all();
         }
 
         const mutex& get_mutex (
@@ -225,13 +122,7 @@ namespace dlib
 
     private:
 
-        mutable HANDLE hSemaphore;
-
-        mutable unsigned long waiters;
-        mutex waiters_mutex;
-        
-
-        mutable HANDLE hCountSema;
+        mutable std::condition_variable cv;
 
         const mutex& m;