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();