cmake: add option to workaround issue observed with old CMake

CMake message contains this: "Invalid escape sequence \"

cmake/OpenCVDownload.cmake

@@ -156,11 +156,13 @@ function(ocv_download)
          TIMEOUT 600
          STATUS status
          LOG __log)
+    if(NOT OPENCV_SKIP_FILE_DOWNLOAD_DUMP)  # workaround problem with old CMake versions: "Invalid escape sequence"
       string(LENGTH "${__log}" __log_length)
       if(__log_length LESS 65536)
         string(REPLACE "\n" "\n# " __log "${__log}")
         ocv_download_log("# ${__log}\n")
       endif()
+    endif()
     if(NOT status EQUAL 0)
       set(msg_level FATAL_ERROR)
       if(DEFINED DL_STATUS)