fixed some warnings

GPU: TargetArchs -> added FEATURE_SET prefix.

samples/cpp/convexhull.cpp

@@ -13,7 +13,7 @@ void help()
 		 << "./convexhull\n" << endl;
 }
 
-int main( int argc, char** argv )
+int main( int /*argc*/, char** /*argv*/ )
 {
     Mat img(500, 500, CV_8UC3);
     RNG& rng = theRNG();

samples/cpp/demhist.cpp

@@ -12,7 +12,7 @@ int _contrast = 100;
 Mat image;
 
 /* brightness/contrast callback function */
-void updateBrightnessContrast( int arg, void* )
+void updateBrightnessContrast( int /*arg*/, void* )
 {
     int histSize = 64;
     int brightness = _brightness - 100;

samples/cpp/squares.cpp

@@ -139,7 +139,7 @@ void drawSquares( Mat& image, const vector<vector<Point> >& squares )
 }
 
 
-int main(int argc, char** argv)
+int main(int /*argc*/, char** /*argv*/)
 {
     static const char* names[] = { "pic1.png", "pic2.png", "pic3.png",
         "pic4.png", "pic5.png", "pic6.png", 0 };