Merge pull request opencv#13709 from michal-kowalczyk:flush-processed-samples-number

* Add flushing to printing number of processed positive/negative samples so that user is informed of updates quickly.

* use flush C api instead of C++ api

Author: michal-kowalczyk

apps/traincascade/cascadeclassifier.cpp

@@ -341,6 +341,7 @@ int CvCascadeClassifier::fillPassedSamples( int first, int count, bool isPositiv
             {
                 getcount++;
                 printf("%s current samples: %d\r", isPositive ? "POS":"NEG", getcount);
+                fflush(stdout);
                 break;
             }
         }