Merge pull request #13253 from alalek:fix_13201

* cmake: install 'legacy/constants_c.h' files

* samples: add compatibility test code

samples/cpp/tutorial_code/compatibility/compatibility_test.cpp

@@ -0,0 +1,19 @@
+#include <iostream>
+
+#include <opencv2/core.hpp>
+
+#include <opencv2/imgcodecs/legacy/constants_c.h>
+#include <opencv2/videoio/legacy/constants_c.h>
+#include <opencv2/photo/legacy/constants_c.h>
+#include <opencv2/video/legacy/constants_c.h>
+
+using namespace cv;
+
+int main(int /*argc*/, const char** /*argv*/)
+{
+    std::cout
+        << (int)CV_LOAD_IMAGE_GRAYSCALE
+        << (int)CV_CAP_FFMPEG
+        << std::endl;
+    return 0;
+}