mesa: don't specify spirv2dxil output if it's not built

[mvnetbiz]

If mesa was overridden to specify a list of gallium drivers that didn't include d3d12, nix would fail the build for not producing the spirv2dxil output.

Things done

For example this now works, where before it failed with failed to produce output path for output 'spirv2dxil'.

nix-build  --expr '((import ./. {}).pkgs.mesa.override { galliumDrivers = [ "llvmpipe" "radeonsi" "i915" ]; vulkanDrivers = [ "swrast" ]; }).drivers'

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[marcusramberg]

Noticed this when updating asahi mesa to latest from Nixpkgs as well.

[mvnetbiz]

It used to be conditional, but was removed in #319047. It looks like the output may also be produced if microsoft-experimental" is in vulkanDrivers.

[mvnetbiz]

If you're trying to make a minimal mesa, you probably have to overrideAttrs a bunch of other stuff too, so maybe just fix it that way instead of adding this back?

let
  pkgs = import ./. { };
  inherit (pkgs) lib;
in
((pkgs.mesa.override {
  galliumDrivers = [ "llvmpipe" ];
  vulkanDrivers = [ "swrast" ];
  libvdpau = null;
}).overrideAttrs (old: {
  outputs = lib.remove "spirv2dxil" old.outputs; # relevant line
  mesonFlags = old.mesonFlags ++ [
    (lib.mesonEnable "gallium-vdpau" false)
    (lib.mesonEnable "gallium-va" false)
    (lib.mesonEnable "gallium-xa" false)
    (lib.mesonBool "gallium-nine" false)
  ];
})).drivers