cmake: clean up external project logic for vulkan-shaders-gen

[bandoti]

This PR cleans up the the ExternalProject_Add logic for vulkan-shaders-gen when multi-configuration generators are used.

These changes expand on #14047, which is to fix an issue causing the llama-cpp-rs rust bindings to no longer compile for vulkan, see: utilityai/llama-cpp-rs#747

[0cc4m]

This is unrelated to this PR, but since we went with the external project logic, cmake doesn't recognize file changes in shaders properly anymore. Sometimes shaders aren't rebuilt, which can be annoying to figure out. You can resolve using "--clean-first", but previously it was working fine. Is this something we can fix?

[bandoti]

Oh no! That's not good.

At first glance I notice that _ggml_vk_shader_deps globs the shader files and then the set command is used afterward, appending the vulkan-shaders-gen targets.

My first guess is to separate those dependencies because it might be messing with the list.

Another option is that we could regenerate the list of globbed files at build time instead of generate-time using the glob flag:

Added in version 3.12: If the CONFIGURE_DEPENDS flag is specified, CMake will add logic to the main build system check target to rerun the flagged GLOB commands at build time. If any of the outputs change, CMake will regenerate the build system.

[bandoti]

I am happy to put the fix in with this one as it is related—and small enough. Thanks for letting me know. 😊

[bandoti]

@0cc4m I added you as a reviewer primarily to check that the shader re-compile works. I spot tested on my system and it seems to rebuild properly when the shaders get modified. Thanks!

[0cc4m]

Thank you, I couldn't reproduce the issue currently so I can't say for certain if this fixes it, but I'll watch it.

[AsbjornOlling]

I tested this by running nix build 'github:bandoti/llama.cpp/cmake-vulkan-multi-config#vulkan'

This compiles without error, unlike the current master branch.

For comparison, running nix build 'github:ggml-org/llama.cpp/b5679#vulkan' fails.

[bandoti]

@AsbjornOlling Thanks for the contribution/help fixing the issue. 😊