QT配置cmake 报错：Cannot specify link libraries for target " XXX XXX " which is not built by this project.或The keyword signature for target_link_libraries has already been used with the target

报错 Cannot specify link libraries for target " XXX XXX " which is not built by this project
是因为在使用target_link_libraries的时候，没有放到add_executable后面.
然而qt项目中的cmakelist.txt已经自动帮你写过add_executable了，所以如果自己加上add_executable又会报错有同名文件被添加。

特别注意一个项目中所有链接库要么都是Private要么都不是，如蓝色框框所示，否则会报错
The keyword signature for target_link_libraries has already been used with the target

顺便提一下cmake编译opencv源码

cmake-gui中选择本地编译器，.../qt/tools/mingwxx/bin中，c选择gcc.exe,c++选择g++.exe，选择advance模式，搜索cmake_make_program选择mingw32make.exe，都在前面所说的路径下,并把.../qt/tools/mingwxx/bin添加到系统环境变量Path中，提前下载opencv_videoio_ffpeg.dll并放入opencv4.6.0\sources\3rdparty\ffmpeg下