cuda: WITH_CUDA=OFF by default (need to enable it explicitly)

cmake/OpenCVFindLibsPerf.cmake

@@ -42,6 +42,12 @@ endif(WITH_IPP_A)
 # --- CUDA ---
 if(WITH_CUDA)
   include("${OpenCV_SOURCE_DIR}/cmake/OpenCVDetectCUDA.cmake")
+  if(NOT HAVE_CUDA)
+    message(WARNING "OpenCV is not able to find/confidure CUDA SDK (required by WITH_CUDA).
+CUDA support will be disabled in OpenCV build.
+To eliminate this warning remove WITH_CUDA=ON CMake configuration option.
+")
+  endif()
 endif(WITH_CUDA)
 
 # --- Eigen ---