Merge pull request #13947 from JoeHowse:4.0.1-patches

* core, stitching: revise syntax to support Visual C++ 2013

* stitching: revise syntax again to support Visual C++ 2013 and other compilers

* stitching: minor update to clarify changes

modules/stitching/src/stitcher.cpp

@@ -44,6 +44,12 @@
 
 namespace cv {
 
+#if __cplusplus >= 201103L || (defined(_MSC_VER) && _MSC_VER >= 1900/*MSVS 2015*/)
+// Stitcher::ORIG_RESOL is initialized in stitching.hpp.
+#else
+const double Stitcher::ORIG_RESOL = -1.0;
+#endif
+
 Ptr<Stitcher> Stitcher::create(Mode mode)
 {
     Ptr<Stitcher> stitcher = makePtr<Stitcher>();