Fixed compilation warnings in samples:

- use per-module includes instead of "opencv.hpp"
- unused function warnings
- undefined macro warning

samples/cpp/videowriter_basic.cpp

@@ -5,7 +5,9 @@
   @date Aug 24, 2016
 */
 
-#include <opencv2/opencv.hpp>
+#include <opencv2/core.hpp>
+#include <opencv2/videoio.hpp>
+#include <opencv2/highgui.hpp>
 #include <iostream>
 #include <stdio.h>
 
@@ -62,4 +64,4 @@ int main(int, char**)
     }
     // the videofile will be closed and released automatically in VideoWriter destructor
     return 0;
-}
+}