message: add message if no OpenCV was found
diff: @@ -39,6 +39,8 @@ endif()
  find_package(OpenCV QUIET)
  if(OpenCV_FOUND)
  list(APPEND EXAMPLES_LIST heat-equation)
  +else()
  + message(STATUS "No OpenCV found, disabling heat-equation example")
  endif()
  if(GINKGO_HAVE_PAPI_SDE)
  