Build system: Allow specifying Win32 resource include paths using LazyPath

lib/std/Build/Module.zig

@@ -110,11 +110,18 @@ pub const RcSourceFile = struct {
     ///  /x (ignore the INCLUDE environment variable)
     ///  /D_DEBUG or /DNDEBUG depending on the optimization mode
     flags: []const []const u8 = &.{},
+    /// Include paths that may or may not exist yet and therefore need to be
+    /// specified as a LazyPath. Each path will be appended to the flags
+    /// as `/I <resolved path>`.
+    include_paths: []const LazyPath = &.{},
 
     pub fn dupe(self: RcSourceFile, b: *std.Build) RcSourceFile {
+        const include_paths = b.allocator.alloc(LazyPath, self.include_paths.len) catch @panic("OOM");
+        for (include_paths, self.include_paths) |*dest, lazy_path| dest.* = lazy_path.dupe(b);
         return .{
             .file = self.file.dupe(b),
             .flags = b.dupeStrings(self.flags),
+            .include_paths = include_paths,
         };
     }
 };
@@ -503,6 +510,9 @@ pub fn addWin32ResourceFile(m: *Module, source: RcSourceFile) void {
     rc_source_file.* = source.dupe(b);
     m.link_objects.append(allocator, .{ .win32_resource_file = rc_source_file }) catch @panic("OOM");
     addLazyPathDependenciesOnly(m, source.file);
+    for (source.include_paths) |include_path| {
+        addLazyPathDependenciesOnly(m, include_path);
+    }
 }
 
 pub fn addAssemblyFile(m: *Module, source: LazyPath) void {

lib/std/Build/Step/Compile.zig

@@ -1281,7 +1281,7 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
                     .win32_resource_file => |rc_source_file| l: {
                         if (!my_responsibility) break :l;
 
-                        if (rc_source_file.flags.len == 0) {
+                        if (rc_source_file.flags.len == 0 and rc_source_file.include_paths.len == 0) {
                             if (prev_has_rcflags) {
                                 try zig_args.append("-rcflags");
                                 try zig_args.append("--");
@@ -1292,6 +1292,10 @@ fn make(step: *Step, prog_node: *std.Progress.Node) !void {
                             for (rc_source_file.flags) |arg| {
                                 try zig_args.append(arg);
                             }
+                            for (rc_source_file.include_paths) |include_path| {
+                                try zig_args.append("/I");
+                                try zig_args.append(include_path.getPath2(module.owner, step));
+                            }
                             try zig_args.append("--");
                             prev_has_rcflags = true;
                         }

test/standalone/windows_resources/build.zig

@@ -10,18 +10,21 @@ pub fn build(b: *std.Build) void {
         .abi = .gnu,
     });
 
-    add(b, b.host, .any, test_step);
-    add(b, target, .any, test_step);
+    const generated_h_step = b.addWriteFile("generated.h", "#define GENERATED_DEFINE \"foo\"");
 
-    add(b, b.host, .gnu, test_step);
-    add(b, target, .gnu, test_step);
+    add(b, b.host, .any, test_step, generated_h_step);
+    add(b, target, .any, test_step, generated_h_step);
+
+    add(b, b.host, .gnu, test_step, generated_h_step);
+    add(b, target, .gnu, test_step, generated_h_step);
 }
 
 fn add(
     b: *std.Build,
     target: std.Build.ResolvedTarget,
     rc_includes: enum { any, gnu },
     test_step: *std.Build.Step,
+    generated_h_step: *std.Build.Step.WriteFile,
 ) void {
     const exe = b.addExecutable(.{
         .name = "zig_resource_test",
@@ -32,6 +35,9 @@ fn add(
     exe.addWin32ResourceFile(.{
         .file = b.path("res/zig.rc"),
         .flags = &.{"/c65001"}, // UTF-8 code page
+        .include_paths = &.{
+            .{ .generated = &generated_h_step.generated_directory },
+        },
     });
     exe.rc_includes = switch (rc_includes) {
         .any => .any,

test/standalone/windows_resources/res/zig.rc

@@ -1,3 +1,7 @@
+// This include file is generated via build.zig, and it #defines GENERATED_DEFINE
+#include "generated.h"
+FOO RCDATA { GENERATED_DEFINE }
+
 #define ICO_ID 1
 
 // Nothing from windows.h is used in this .rc file,