meson: Check for bundled Vulkan/SPIR-V headers

[flibitijibibo]

This is at least enough to use the system headers when building from the tarball, but doesn't check for any version requirements. While imperfect, it's an improvement on the current solution which is to sed -i meson.build, which is sort of overkill...

Partially fixes #3322

[misyltoad]

There is no reason to ever use system headers for DXVK

[flibitijibibo]

There is exactly one reason, and it's one only distributors care about: When building packages they have to use release tarballs, and GitHub still doesn't include the submodules. If dxvk had a custom archive for each release that mimics a recursive clone and trims out all the .git files, that would obsolete this change. (FNA does this, it's not too painful to automate)

[flibitijibibo]

For completion's sake, this is the FNA "archivinate.sh" script:

#!/bin/bash

set -ex

cd "`dirname "$0"`"

git clone --recursive https://github.com/FNA-XNA/FNA
rm -rfv FNA/.git* FNA/lib/*/.git* FNA/lib/FNA3D/*/.git*

This could totally be made into a generic script by reading .gitmodules files recursively, but for projects that know their submodule tree a quick script like this does the job!