1.对cv::Mat::updateContinuityFlag()未定义引用
orb-slam2的CMakeLists.txt里默认查找了ros里自带的opencv，所以改为
find_package(OpenCV 3.4.5 REQUIRED)
即可使用自己安的高版本opencv啦～
2.usleep未定义：
对下列文件增加
#include<unistd.h>
Examples/Monocular/mono_euroc.cc
Examples/Monocular/mono_kitti.cc
Examples/Monocular/mono_tum.cc
Examples/RGB-D/rgbd_tum.cc
Examples/Stereo/stereo_euroc.cc
Examples/Stereo/stereo_kitti.cc
src/LocalMapping.cc
src/LoopClosing.cc
src/System.cc
src/Tracking.cc
src/Viewer.cc