Updated to zig v0.15

- Added manifest.scm

.envrc

@@ -0,0 +1 @@
+use guix

build.zig

@@ -4,13 +4,18 @@ pub fn build(b: *std.Build) void {
     const target = b.standardTargetOptions(.{});
     const optimize = b.standardOptimizeOption(.{});
 
-    const lib = b.addStaticLibrary(.{
-        .name = "vulkan-headers",
-        .root_source_file = b.addWriteFiles().add("empty.c", ""),
+    const mod = b.addModule("vulkan-headers", .{
         .target = target,
         .optimize = optimize,
     });
 
+    const lib = b.addLibrary(.{
+        .name = "vulkan-headers",
+        .linkage = .static,
+        .root_module = mod,
+    });
+    lib.addCSourceFile(.{ .file = b.addWriteFiles().add("empty.c", "") });
+
     inline for (.{ "vk_video", "vulkan" }) |subdir| {
         lib.installHeadersDirectory(b.path("include/" ++ subdir), subdir, .{});
     }

manifest.scm

@@ -0,0 +1,5 @@
+;; What follows is a "manifest" equivalent to the command line you gave.
+;; You can store it in a file that you may then pass to any 'guix' command
+;; that accepts a '--manifest' (or '-m') option.
+
+(specifications->manifest (list "zig@0.15" "git" "zig-zls@0.15"))