Merge pull request #16125 from alalek:core_safe_xadd

modules/core/include/opencv2/core/cvdef.h

@@ -525,7 +525,11 @@ Cv64suf;
 #  include <intrin.h>
 #  define CV_XADD(addr, delta) (int)_InterlockedExchangeAdd((long volatile*)addr, delta)
 #else
-   CV_INLINE CV_XADD(int* addr, int delta) { int tmp = *addr; *addr += delta; return tmp; }
+  #ifdef OPENCV_FORCE_UNSAFE_XADD
+    CV_INLINE CV_XADD(int* addr, int delta) { int tmp = *addr; *addr += delta; return tmp; }
+  #else
+    #error "OpenCV: can't define safe CV_XADD macro for current platform (unsupported). Define CV_XADD macro through custom port header (see OPENCV_INCLUDE_PORT_FILE)"
+  #endif
 #endif