CMake dependencies for auto-generated code

Question

I have an existing project that is using CMake and has a complex binary target (compiles many .c file that include many .h files).

I want to add a process to auto generate some code before the existing target is built. So let's say the target is X, and the auto-generated process is Y. I want Y to run before X is built, but the auto-generating process and build take a long time, so I don't want them to run every time, I only want it to run when something changes.

The problem I see is that X is dependent Y and Y is dependent on X. Is there a way to solve this?

I basically want to say that the auto-generated process (Y) should be dependent on all the source files (.c and .h files) for target X (but not the build result), and that the the target X should depend on the result of Y.

Is there some way of doing this?

Note that I found that you can use glob to generate dependency on all the .c and .h files in the project, but the downside of that is that if someone adds a .h file they don't have to update Cmake, and so the Cmake won't re-run again before make, which means that we could miss the dependency on the header file and get bad results due to stale auto-generated code.