Use Clang -Wc++17-extensions -Wc++20-extensions

Fix the remaining use of C++20 designated initializers.

Change-Id: Id83fd4f9129d9bd26084c12160b24423d763a428
(cherry picked from commit 2513cda964a3a0db5c7dbb51d531e0b844f4182b)
diff --git a/build/cmake/aom_configure.cmake b/build/cmake/aom_configure.cmake
index 1279ad3..26c521e 100644
--- a/build/cmake/aom_configure.cmake
+++ b/build/cmake/aom_configure.cmake
@@ -314,6 +314,8 @@
   add_compiler_flag_if_supported("-Wuninitialized")
   add_compiler_flag_if_supported("-Wunused")
   add_compiler_flag_if_supported("-Wvla")
+  add_cxx_flag_if_supported("-Wc++17-extensions")
+  add_cxx_flag_if_supported("-Wc++20-extensions")
 
   if(CMAKE_C_COMPILER_ID MATCHES "GNU" AND SANITIZE MATCHES "address|undefined")
 
diff --git a/test/ratectrl_qmode_test.cc b/test/ratectrl_qmode_test.cc
index 13ebd0e..d950626 100644
--- a/test/ratectrl_qmode_test.cc
+++ b/test/ratectrl_qmode_test.cc
@@ -1028,7 +1028,7 @@
   const auto gop_info = rc.DetermineGopInfo(firstpass_info);
   ASSERT_THAT(gop_info.status(), IsOkStatus());
   const GopStructList &gop_list = *gop_info;
-  const aom_rational_t frame_rate = { .num = 30, .den = 1 };
+  const aom_rational_t frame_rate = { 30, 1 };
   const aom::VideoInfo input_video = {
     kFrameWidth, kFrameHeight,
     frame_rate,  AOM_IMG_FMT_I420,