Vulkan detection fails on msys build

[benjamin-otte]

When installing the mingw-w64-x86_64-vulkan and mingw-w64-x86_64-vulkan-loader packages but forgetting the mingw-w64-x86_64-vulkan-headers package, meson will behave as if Vulkan is available but then the build will fail because it can't #include <vulkan/vulkan.h>.

It should set the Vulkan dependency as not-found.

This was found after adding Vulkan support for GTK4 CI builder. See https://gitlab.gnome.org/GNOME/gtk/-/jobs/3737222 for an example build.
Adding the missing packages fixed the build.

[lazka]

Duplicate of #7956 it seems

To re-iterate it here, the problem is that the pkg-config file is part of the vulkan-loader project: https://github.com/KhronosGroup/Vulkan-Loader/blob/main/loader/vulkan.pc.in the header files are in vulkan-headers: https://github.com/KhronosGroup/Vulkan-Headers.

So using the vulkan dependency to check for the headers is likely not correct (??)

As for MSYS2, we packaging them separately, same as Arch Linux.

Three options:

• Gtk checks for the existence of the headers in meson.build in addition for the vulkan dep
• in MSYS2, we make vulkan-loader depend on vulkan-headers
• in MSYS2, we merge vulkan-loader and vulkan-headers into one package, similar to what Debian did in 2019

[lazka]

I've asked upstream if they have an opinion: KhronosGroup/Vulkan-Loader#1463