Added scope qualifiers to some mutex objects to avoid a name collision

in newer versions of clang.

examples/parallel_for_ex.cpp

@@ -148,7 +148,7 @@ void example_using_lambda_functions()
     // parallel for loop is implemented using threads, all the usual techniques for
     // ensuring thread safety can be used. 
     int sum = 0;
-    mutex m;
+    dlib::mutex m;
     vect.assign(10, 2);
     parallel_for(num_threads, 0, vect.size(), [&](long i){
         // The sleep statements still execute in parallel.  
@@ -194,7 +194,7 @@ struct function_object_sum
 
     const std::vector<int>& vect;
     int& sum;
-    mutex m;
+    dlib::mutex m;
 
     void operator() (long i) const
     {

examples/threads_ex.cpp

@@ -23,7 +23,7 @@ using namespace std;
 using namespace dlib;
 
 int thread_count = 10;
-mutex count_mutex; // This is a mutex we will use to guard the thread_count variable.  Note that the mutex doesn't know
+dlib::mutex count_mutex; // This is a mutex we will use to guard the thread_count variable.  Note that the mutex doesn't know
                    // anything about the thread_count variable.  Only our usage of a mutex determines what it guards.  
                    // In this case we are going to make sure this mutex is always locked before we touch the
                    // thread_count variable.