Fixed compilation warnings in samples:

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

samples/cpp/detect_mser.cpp

@@ -1,6 +1,8 @@
-#include <opencv2/opencv.hpp>
+#include <opencv2/core.hpp>
+#include <opencv2/imgproc.hpp>
+#include <opencv2/highgui.hpp>
+#include <opencv2/features2d.hpp>
 #include "opencv2/core/opengl.hpp"
-#include "opencv2/cvconfig.h"
 
 #include <vector>
 #include <map>