Version check in build assumes a fully built and installed copy of Vulkan-headers

[zoddicus]

In the build rules, there is a version check here that assumes a fully built and installed copy of Vulkan-Headers. Specifically it uses find_package to get the version and if the Header isn't installed the value for VulkanHeaders_VERSION will be undefined, which leads to a syntax error. This creates a hard workflow dependency where it is expected that Vulkan-headers is configured, built and installed before configuring Vulkan-Loader.

This becomes an issue for Dawn, since our CMake rules use add_subdirectory on the various dependencies to configure all of the dependencies and then build all of the dependencies as part of the build instead of having them individually built/installed beforehand.

Since the VulkanHeaders haven't been built and installed at configure time, this causes the above mentioned syntax error. This can be worked around, patch, by just defining a faux value for the version.

I think the Vulkan-Loader build rules could be adjusted to support our build flow without causing breaking changes to existing users. Specifically by either adding a guard to the check to ensure that VulkanHeaders_VERSION is defined in the conditional, or by skipping the entire check if Vulkan::Headers is already present, i.e. if someone is building using add_subdirectory they are responsible for ensuring the version is good.

[charles-lunarg]

I can confirm this issue in a quickly created test project that clones Vulkan-Headers & Vulkan-Loader, then does add_subdirectory() on them.

Since the issue is the variable not being defined, I added a check for it like you suggested. Will merge once its passed CI.