gpuoptflow module fixes

modules/gpuoptflow/include/opencv2/gpuoptflow.hpp

@@ -43,12 +43,14 @@
 #ifndef __OPENCV_GPUOPTFLOW_HPP__
 #define __OPENCV_GPUOPTFLOW_HPP__
 
+#ifndef __cplusplus
+#  error gpuoptflow.hpp header must be compiled as C++
+#endif
+
 #include "opencv2/core/gpumat.hpp"
 
 namespace cv { namespace gpu {
 
-////////////////////////////////// Optical Flow //////////////////////////////////////////
-
 class CV_EXPORTS BroxOpticalFlow
 {
 public:
@@ -282,7 +284,6 @@ private:
     GpuMat extended_I1;
 };
 
-
 //! Interpolate frames (images) using provided optical flow (displacement field).
 //! frame0   - frame 0 (32-bit floating point images, single channel)
 //! frame1   - frame 1 (the same type and size)