diff --git a/llvm/utils/gn/secondary/libcxx/include/BUILD.gn b/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
index f1f1f37686e2d5..4d7cdbefb2e600 100644
--- a/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
+++ b/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
@@ -1,52 +1,68 @@
-import("//clang/resource_dir.gni")
 import("//libcxx/config.gni")
 import("//llvm/utils/gn/build/write_cmake_config.gni")
 
-libcxx_needs_site_config =
-    libcxx_abi_version != 1 || libcxx_abi_namespace != "" || libcxx_abi_unstable
+write_cmake_config("write_config") {
+  input = "__config_site.in"
+  output = "$target_gen_dir/__config_site"
 
-if (libcxx_needs_site_config) {
-  write_cmake_config("write_config") {
-    input = "__config_site.in"
-    output = "$target_gen_dir/__config_site"
-
-    values = []
-    if (libcxx_abi_version != 1) {
-      values += [ "_LIBCPP_ABI_VERSION=$libcxx_abi_version" ]
-    }
-    if (libcxx_abi_namespace != "") {
-      values += [ "_LIBCPP_ABI_NAMESPACE=$libcxx_abi_namespace" ]
-    }
-    if (libcxx_abi_unstable) {
-      values += [ "_LIBCPP_ABI_UNSTABLE=1" ]
-    }
+  values = [
+    "_LIBCPP_ABI_FORCE_ITANIUM=",
+    "_LIBCPP_ABI_FORCE_MICROSOFT=",
+    "_LIBCPP_HIDE_FROM_ABI_PER_TU_BY_DEFAULT=",
+    "_LIBCPP_HAS_NO_GLOBAL_FILESYSTEM_NAMESPACE=",
+    "_LIBCPP_HAS_NO_STDIN=",
+    "_LIBCPP_HAS_NO_STDOUT=",
+    "_LIBCPP_HAS_NO_THREADS=",
+    "_LIBCPP_HAS_NO_MONOTONIC_CLOCK=",
+    "_LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS=",
+    "_LIBCPP_HAS_MUSL_LIBC=",
+    "_LIBCPP_HAS_THREAD_API_PTHREAD=",
+    "_LIBCPP_HAS_THREAD_API_EXTERNAL=",
+    "_LIBCPP_HAS_THREAD_API_WIN32=",
+    "_LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL=",
+    "_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS=",
+    "_LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS=1",
+    "_LIBCPP_NO_VCRUNTIME=",
+    "_LIBCPP_TYPEINFO_COMPARISON_IMPLEMENTATION=",
+    "_LIBCPP_ABI_NAMESPACE=",
+    "_LIBCPP_HAS_PARALLEL_ALGORITHMS=",
+    "_LIBCPP_HAS_NO_RANDOM_DEVICE=",
+    "_LIBCPP_HAS_NO_LOCALIZATION=",
+    "_LIBCPP_ABI_DEFINES=",
+  ]
+  if (libcxx_abi_version != 1) {
+    values += [ "_LIBCPP_ABI_VERSION=$libcxx_abi_version" ]
+  } else {
+    values += [ "_LIBCPP_ABI_VERSION=" ]
   }
-
-  # Generate a custom __config header. The new header is created
-  # by prepending __config_site to the current __config header.
-  action("concat_config") {
-    script = "//libcxx/utils/cat_files.py"
-    inputs = [
-      "$target_gen_dir/__config_site",
-      "__config",
-    ]
-    outputs = [ "$target_gen_dir/__config" ]
-    args = [
-      "$target_gen_dir/__config_site",
-      "__config",
-      "-o",
-      "$target_gen_dir/__config",
-    ]
-    deps = [ ":write_config" ]
+  if (libcxx_abi_namespace != "") {
+    values += [ "_LIBCPP_ABI_NAMESPACE=$libcxx_abi_namespace" ]
   }
-
-  copy("copy_config") {
-    sources = [ "$target_gen_dir/__config" ]
-    outputs = [ "$clang_resource_dir/include/c++/v1/{{source_file_part}}" ]
-    deps = [ ":concat_config" ]
+  if (libcxx_abi_unstable) {
+    values += [ "_LIBCPP_ABI_UNSTABLE=1" ]
+  } else {
+    values += [ "_LIBCPP_ABI_UNSTABLE=" ]
   }
 }
 
+# Generate a custom __config header. The new header is created
+# by prepending __config_site to the current __config header.
+action("concat_config") {
+  script = "//libcxx/utils/cat_files.py"
+  inputs = [
+    "$target_gen_dir/__config_site",
+    "__config",
+  ]
+  outputs = [ "$root_build_dir/include/c++/v1/__config" ]
+  args = [
+    rebase_path("$target_gen_dir/__config_site", root_build_dir),
+    rebase_path("__config", root_build_dir),
+    "-o",
+    rebase_path(outputs[0], root_build_dir)
+  ]
+  deps = [ ":write_config" ]
+}
+
 copy("include") {
   sources = [
     "__availability",
@@ -235,7 +251,7 @@ copy("include") {
     "wchar.h",
     "wctype.h",
   ]
-  deps = []
+  deps = [ ":concat_config" ]
   if (target_os != "mac" && target_os != "win") {
     # libcxx/cmake/Modules/HandleLibCXXABI.cmake sets
     # LIBCXX_CXX_ABI_HEADER_TARGET if the libcxx abi library either of
@@ -247,10 +263,5 @@ copy("include") {
     # don't get copied on macOS due to that.
     deps += [ "//libcxxabi/include" ]
   }
-  if (!libcxx_needs_site_config) {
-    sources += [ "__config" ]
-  } else {
-    deps += [ ":copy_config" ]
-  }
   outputs = [ "$root_build_dir/include/c++/v1/{{source_target_relative}}" ]
 }