         .target(
             name: "llbuildBasic",
             dependencies: ["llvmSupport"],
-            path: "lib/Basic"
+            path: "lib/Basic",
+            cxxSettings: [
+              .define("LLVM_ON_WIN32", .when(platforms: [.windows])),
+            ]
         ),
         .target(
             name: "llbuildCore",

        .target(
            name: "libllbuild",
            dependencies: ["llbuildCore", "llbuildBuildSystem"],
            path: "products/libllbuild"
        ),

        /// The public llbuild Swift API.

// swift-tools-version:5.2

// This file defines Swift package manager support for llbuild. See:
//  https://github.com/apple/swift-package-manager/tree/master/Documentation
