Update build.zig for 0.16 fs.Dir -> Io.Dir (#31)

Co-authored-by: Jay Petacat <jay@jayschwa.net>

Author: mdm5995

build.zig

@@ -67,13 +67,22 @@ pub fn build(b: *std.Build) void {
             const cache_include = std.fs.path.join(b.allocator, &.{ b.sysroot.?, "cache", "sysroot", "include" }) catch @panic("Out of memory");
             defer b.allocator.free(cache_include);
 
-            // TODO: Remove compatibility shim when Zig 0.16.0 is the minimum required version.
-            const open_dir_opts: std.fs.Dir.OpenOptions = if (@hasField(std.fs.Dir.OpenOptions, "follow_symlinks"))
-                .{ .access_sub_paths = true, .follow_symlinks = false }
-            else
-                .{ .access_sub_paths = true, .no_follow = true };
-            var dir = std.fs.openDirAbsolute(cache_include, open_dir_opts) catch @panic("No emscripten cache. Generate it!");
-            dir.close();
+            // TODO: Remove compatibility shim when minimum Zig version is 0.16.0.
+            if (@hasDecl(std.Io, "Dir")) {
+                // Zig 0.16.0-dev
+                var dir = std.Io.Dir.openDirAbsolute(mod.owner.graph.io, cache_include, .{
+                    .access_sub_paths = true,
+                    .follow_symlinks = false,
+                }) catch @panic("No emscripten cache. Generate it!");
+                dir.close(mod.owner.graph.io);
+            } else if (@hasDecl(std.fs, "Dir")) {
+                // Zig 0.15.x
+                var dir = std.fs.openDirAbsolute(cache_include, .{
+                    .access_sub_paths = true,
+                    .no_follow = true,
+                }) catch @panic("No emscripten cache. Generate it!");
+                dir.close();
+            } else @compileError("Fix `openDirAbsolute` compatibility shim");
 
             mod.addIncludePath(.{ .cwd_relative = cache_include });
         },