Option to build with system headers for Linux distribution packages

[enunes]

I attempted to create a package for this to submit to upstream Fedora, but I'm hitting an issue that this project currently depends on the external dependencies from git submodules. Distributions like Fedora usually do not use bundled / downloaded subdependencies but use project options to use distribution packages as dependencies instead.

Right now even if I have the latest Vulkan headers from the distrubition, build fails with:

-- Vulkan Header = VULKAN_HEADER-NOTFOUND
CMake Error at cmake/FindVulkanVersion.cmake:30 (file):
  file STRINGS file
  "<...>/gfxreconstruct/VULKAN_HEADER-NOTFOUND" cannot be read.
Call Stack (most recent call first):
  CMakeLists.txt:115 (include)

Apparently because of

gfxreconstruct/cmake/FindVulkanVersion.cmake

Line 12 in 544fb67

external/Vulkan-Headers/include/vulkan

I see that Debian has also had to patch the same thing in order to build their package:
https://salsa.debian.org/xorg-team/vulkan/gfxreconstruct/-/blob/master/debian/patches/01_Use_system_Vulkan.patch?ref_type=heads

More recently there are more hard includes of external/ from git submodules like external/SPIRV-Reflect which I think would also make this more difficult.

I wanted to prepare and send a patch to propose a fix for this, but unfortunately the CMake is a bit hard to follow here.

Could we get an option to attempt to look for and use system libraries first, that we can use in distrubution packages? For example something like an UPDATE_DEPS (default OFF) as seen in other Vulkan packages like https://github.com/KhronosGroup/Vulkan-Tools/blob/main/BUILD.md#dependencies

Thanks!