Add _WIN32 as a predefined macro

This is not mentioned in https://learn.microsoft.com/en-us/windows/win32/menurc/predefined-macros but it is empirically defined by default in the Win32 RC compiler.

src/preprocess.zig

@@ -96,6 +96,7 @@ pub fn appendAroArgs(arena: Allocator, argv: *std.ArrayList([]const u8), options
         "--emulate=msvc",
         "-nostdinc",
         "-DRC_INVOKED",
+        "-D_WIN32", // undocumented, but defined by default
     });
     for (options.extra_include_paths.items) |extra_include_path| {
         try argv.append("-I");

test/cli.zig

@@ -17,6 +17,25 @@ test "including windows.h" {
     try testExpectNoError(tmp.dir, &.{ exe_path, "/:auto-includes", "gnu", "test.rc" });
 }
 
+test "predefined macros" {
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    try tmp.dir.writeFile(.{
+        .sub_path = "test.rc",
+        .data =
+        \\1 RCDATA { RC_INVOKED, _WIN32 }
+        ,
+    });
+
+    try testExpectNoError(tmp.dir, &.{ exe_path, "test.rc" });
+    // case sensitive, so this doesn't undef RC_INVOKED
+    try testExpectNoError(tmp.dir, &.{ exe_path, "/u", "rc_invoked", "test.rc" });
+    // undefing predefined macros works
+    try testExpectError(tmp.dir, &.{ exe_path, "/u", "RC_INVOKED", "test.rc" });
+    try testExpectError(tmp.dir, &.{ exe_path, "/u", "_WIN32", "test.rc" });
+}
+
 fn testExpectNoError(dir: std.fs.Dir, argv: []const []const u8) !void {
     const allocator = std.testing.allocator;
 
@@ -38,3 +57,25 @@ fn testExpectNoError(dir: std.fs.Dir, argv: []const []const u8) !void {
         return error.UnexpectedErrors;
     }
 }
+
+fn testExpectError(dir: std.fs.Dir, argv: []const []const u8) !void {
+    const allocator = std.testing.allocator;
+
+    const dir_path = try dir.realpathAlloc(allocator, ".");
+    defer allocator.free(dir_path);
+
+    const result = try std.process.Child.run(.{
+        .allocator = allocator,
+        .argv = argv,
+        .max_output_bytes = std.math.maxInt(u32),
+        .cwd = dir_path,
+        .cwd_dir = dir,
+    });
+    defer allocator.free(result.stdout);
+    defer allocator.free(result.stderr);
+
+    if (result.term != .Exited or result.term.Exited == 0) {
+        std.debug.print("command unexpectedly succeeded:\n---\nstdout:\n---\n{s}\n---\nstderr:\n---\n{s}\n", .{ result.stdout, result.stderr });
+        return error.UnexpectedSuccess;
+    }
+}