啊，我现在知道发生了什么：CMake中有一个称为接口库的概念实际上并没有建立一个库二进制文件。它是专为像Eigen这样的仅模板库而设计的。

Ah now I know what's going on: there's a concept called "interface library" in CMake that doesn't actually build a library binary. It is designed precisely for template-only libraries like Eigen.

参考文献：
- https://cmake.org/cmake/help/latest/command/add_library.html#interface-libraries
- http://mariobadr.com/creating-a-header-only-library-with-cmake.html

希望这可以帮助其他挣扎cmake的人！

Hope this helps other souls struggling with cmake!