Enable Clang's -Wc23-extensions warning

See
https://clang.llvm.org/docs/DiagnosticsReference.html#wc23-extensions.

Change-Id: Ie216a274f4f6c7b5dee333dae4264468f28dd6a0

build/cmake/aom_configure.cmake

diff --git a/build/cmake/aom_configure.cmake b/build/cmake/aom_configure.cmake
index 8ab58f1..9b8a0dc 100644
--- a/build/cmake/aom_configure.cmake
+++ b/build/cmake/aom_configure.cmake
@@ -369,6 +369,7 @@
   add_compiler_flag_if_supported("-Wunreachable-code-aggressive")
   add_compiler_flag_if_supported("-Wunused")
   add_compiler_flag_if_supported("-Wvla")
+  add_c_flag_if_supported("-Wc23-extensions")
   add_cxx_flag_if_supported("-Wc++20-extensions")
   add_cxx_flag_if_supported("-Wc++23-extensions")