cmake编译opencv
编译过程出现文件下载失败问题：
face_landmark_model.dat文件下载地址：https://github.com/opencv/opencv_3rdparty/tree/contrib_face_alignment_20170818
修改opencv/opencv_contrib/blob/master/modules/face/CMakeLists.txt，把 "https://raw.githubusercontent.com/opencv/opencv_3rdparty/${__commit_hash}/"
改为 "file:///opencv/download_path/"
ippicv 文件下载地址：https://github.com/opencv/opencv_3rdparty/tree/ippicv/master_20180723/ippicv
修改 /opencv/opencv/blob/master/3rdparty/ippicv/CMakeLists.txt,把“https://github.com/opencv/opencv_3rdparty/tree/ippicv/master_20180723/ippicv” 替换为 “file:///opencv/download_path/”