added optional dependency stitching->nonfree. When nonfree is available, make sure linker does not throw it away.

modules/stitching/CMakeLists.txt

@@ -1,3 +1,3 @@
 set(the_description "Images stitching")
-ocv_define_module(stitching opencv_imgproc opencv_features2d opencv_calib3d opencv_objdetect OPTIONAL opencv_gpu)
+ocv_define_module(stitching opencv_imgproc opencv_features2d opencv_calib3d opencv_objdetect OPTIONAL opencv_gpu opencv_nonfree)
 

modules/stitching/src/matchers.cpp

@@ -50,6 +50,12 @@ using namespace cv::detail;
 using namespace cv::gpu;
 #endif
 
+#ifdef HAVE_OPENCV_NONFREE
+#include "opencv2/nonfree/nonfree.hpp"
+
+static bool makeUseOfNonfree = initModule_nonfree();
+#endif
+
 namespace {
 
 struct DistIdxPair