CMake: Offer multiple choices for Vulkan C headers

[M2-TE]

Using following priority to determine where to get the C headers:

1. Vulkan-Headers subdirectory, if submodule is pulled
2. Vulkan::Headers target, if present (e.g. via FindVulkan or add_subdirectory)
3. Vulkan::Headers target, requiring FindVulkan

[asuessenbach]

@M2-TE Your change generated build failures on gitlab.khronos.org/vulkan:

CMake Error at /usr/share/cmake-3.25/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find Vulkan (missing: Vulkan_LIBRARY Vulkan_INCLUDE_DIR) (found
  version "")
Call Stack (most recent call first):
  /usr/share/cmake-3.25/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
  /usr/share/cmake-3.25/Modules/FindVulkan.cmake:597 (find_package_handle_standard_args)
  CMakeLists.txt:224 (find_package)
-- Configuring incomplete, errors occurred!

Any idea how to resolve that?
For now, I just reverted this by #2080.

[M2-TE]

@asuessenbach Would this not indicate that the Vulkan-Headers submodule has not been pulled? If both this and FindVulkan are missing, how were the Vulkan C headers acquired before?

I sadly do not think I have access to gitlab.khronos.org/vulkan, is it a mirror of Vulkan-Hpp?

[asuessenbach]

is it a mirror of Vulkan-Hpp?

@M2-TE No, it's the Vulkan-Headers build. That is: that directory doesn't exist, yet, at that point in time.
And, after all, the VulkanHppGenerator doesn't really need that include directory. It's just the samples and tests, that need it.

Would you like to change your PR accordingly?

[M2-TE]

the VulkanHppGenerator doesn't really need that include directory

@asuessenbach That is my bad, makes sense in retrospect.

Would you like to change your PR accordingly?

Yea, I'll simply have CMake emit a warning if the include directory appears be missing.