llama.mak: support building Vulkan backend

[Peter0x44]

Requires the Vulkan SDK installed to build.
It can be downloaded from: https://vulkan.lunarg.com/

[Peter0x44]

A good talk on what to expect regarding Vulkan in llama.cpp:

https://fosdem.org/2026/schedule/event/CZSPSC-llama-cpp-vulkan/

I don't love depending on the Vulkan SDK, and nothing it does is really irreplaceable, but upstream expects it, so it's what I used.

[Peter0x44]

to give a bit more overview of what vulkan sdk provides, just in case something is unclear.

glslc - compiler for glsl -> spir-v. Vulkan does not support text shaders, it instead consumes spir-v IR at runtime.
vulkan headers - self explanatory. In my own projects, I prefer volk, which is unofficial.
volk is more-or-less a vulkan equivalent of GLAD. It calls LoadLibrary on vulkan-1.dll which ships as part of Windows, at C:\Windows\System32.
(not that it's used here).

vulkan library - vulkan-1.lib import lib for vulkan-1.dll

Though I have not tried to, these components could all be built and provided separately, in theory (and likely practice).

Here are the official vulkan build instructions, for reference.
https://github.com/ggml-org/llama.cpp/blob/master/docs/build.md#for-windows-users