[libclc] [CMake][libclc] Improve dependencies to avoid build errors (PR #95018)

[Fraser Cormack via cfe-commits]

frasercrmck wrote:

> Yes, the article you link to is highly relevant. Without this PR we're hitting the problem described in example #4.
> 
> We can promote the issue to a build failure by choosing a custom command which can't run concurrently with itself:
> 
> ```cmake
> cmake_minimum_required(VERSION 3.2)
> 
> add_custom_command(
>     OUTPUT gen
>     # Try to grab a lock and fail immediately if we can't:
>     COMMAND flock -x -n gen.lock sleep 1
>     COMMAND cmake -E echo Hello > gen
>     )
> 
> add_custom_target(
>     my-all-1 ALL DEPENDS gen
>     )
> 
> add_custom_target(
>     my-all-2 ALL DEPENDS gen
>     )
> ```
> 
> With the above `make` will succeed, but `make -j2` will fail. The issue is not exposed with Ninja. This is the same behavior I see in libclc today, except more jobs are needed. Adding an intermediate target and depending on both the new target and the file does seem to fix all of our problems.
> 
> I've added the file dependencies back in [b707fc7](https://github.com/llvm/llvm-project/commit/b707fc7dea2e5e57bc3e4f01e27b9ca7bbf0e398), and I think it fixes the build issue while preserving the incremental build behavior you described. Could you please give it a try?

Brilliant, thanks. I have played around and see that incremental builds appear to work as intended. I've tried deleting and `touch`ing various files and full rebuilds and partial rebuilds do what I'd expect.

Thanks also for pointing out I've already read about this exact issue (example #4) and missed it! It's a real shame there isn't a better framework for testing CMake and build generators themselves.

https://github.com/llvm/llvm-project/pull/95018