opencv源码编译出错 IPPICV: Download failed: 7；“Couldn‘t connect to server“

opencv3.4.8源码编译出错 IPPICV: Download failed: 7；“Couldn‘t connect to server“
warning信息

CMake Warning at cmake/OpenCVDownload.cmake:202 (message):
  IPPICV: Download failed: 7;"Couldn't connect to server"

  For details please refer to the download log file:

  /usr/local/opencv-3.4.8/build/CMakeDownloadLog.txt

Call Stack (most recent call first):
  3rdparty/ippicv/ippicv.cmake:42 (ocv_download)
  cmake/OpenCVFindIPP.cmake:243 (download_ippicv)
  cmake/OpenCVFindLibsPerf.cmake:12 (include)
  CMakeLists.txt:681 (include)

这是网络无法下载的原因，可以采用如下方法解决：
1.进入源码目录
opencv-3.4.8/3rdparty/ippicv
2.手动修改ippicv.cmake文件，删掉下载链接，指向ippicv_2019_lnx_intel64_general_20180723.tgz所在目录，该文件下载地址
由于网络下载很慢，可以从这里下载链接

ocv_download(FILENAME ${OPENCV_ICV_NAME}
               HASH ${OPENCV_ICV_HASH}
               URL
                 "${OPENCV_IPP