This just copies the file (as needed) into the current binary directory. Then you add the current binary directory as an include directory. Essentially you are just making a new include directory that doesn’t have dont_include_this.h.
Note that this solution requires a rerun of CMake if include_this.h changes. You’ll want add_custom_command(cmake -E copy_if_different) to do it at build time. However, it is probably better to just split the headers in the source tree in the first place: