Some other file types I have seen in source directories: CMakeLists.txt, .ico files, ...

This is bad practice, since the src directory is meant for sources (and headers). Using it for any other purpose would qualify as bad naming. The CMakeLists.txt should be a sibling of the src directory, not a child. The icons should go to a resource folder along with other similar resources.

We should additionally consider splitting the include and source directories in order to have the same file tree in build (include + source) and install (only include) directories. I would argue this is widespread, de-facto standard CMake/C++ practice by now.