Add dependency to the CMake-generated build-system

In short: I know how to add dependencies to targets, in a CMake-generated build system. But I would like to add dependencies to the generated build-system itself.

Longer question: In the CMake-generated build process of cgal, we would like CMake to automatically re-run the configuration step, when certain files are modified. Unneeded details are hidden below:

As a matter of fact, we generate using CMake the build system for the CGAL libraries/examples/demos, but also at the same time the build system for our Doxygen-generated documentation. The Doxyfile is generated from multiple files.

When the CMake generator is "Makefile", there is a special target in the Makefile, that is named rebuild_cache, but that target (at the Makefile level) is not a CMake-target. And anyway, I look for a solution that is cross-platform, that is: usable with all CMake generators. I have the impression that what I want is not yet doable with CMake. Can you please confirm, so that I can fill a documented feature-request?

Since CMake 3.0, you can add such a file to the directory property CMAKE_CONFIGURE_DEPENDS. This property holds a list of files; if any of them changes, CMake will trigger re-configuration.

Here is a small example. Assuming your Doxyfile is generated from Doxyfile.in.1 and Doxyfile.in.2 in the current source directory, the property could be used like this:

set_property(
  DIRECTORY 
  APPEND 
  PROPERTY CMAKE_CONFIGURE_DEPENDS 
  Doxyfile.in.1
  Doxyfile.in.2
)

If you're using CMake 2.x, the property CMAKE_CONFIGURE_DEPENDS is not available, but you can use the following trick:

Pass the files through configure_file(), even if you just COPYONLY them someplace and don't use the resulting copies. configure_file() introduces precisely the buildsystem dependency you're looking for.

This works, but it adds the overhead of copying the file.

_{(Note: This trick was also the original content of this answer, since I was not aware of CMAKE_CONFIGURE_DEPENDS at time of answering).}