Ubuntu18.04+OpenCV3.2.0源码安装时，出现错误：Failed to download v3.1.0/protobuf-cpp-3.1.0.tar.gz ，解决方法：下载附件zip文件，解压后得到protobuf-cpp-3.1.0.tar.gz文件，替换/opencv_contrib-3.2.0/modules/dnn/.download/bd5e3eed635a8d32e2b99658633815ef/v3.1.0目录下的同名文件，重新cmake编译即可。亲测有效。
opencv官方下载的扩展包，opencv_contrib-3.4.8
Repository for OpenCV's extra modules
This repository is intended for development of so-called "extra" modules,
contributed functionality. New modules quite often do not have stable API,
and they are not well-t
