Add -Wno-c23-extensions globally.

We're switching to C23 as the default anyway, and even code that can't totally switch should still use as many C23 extensions as it wants to.

Change-Id: I8b8b529e39964f94869cf8be0b274adb8bbb25ad
diff --git a/cc/config/global.go b/cc/config/global.go
index d63f86d..36690d6 100644
--- a/cc/config/global.go
+++ b/cc/config/global.go
@@ -72,6 +72,9 @@
 
 		// Warnings disabled by default.
 
+		// We should encourage use of C23 features even when the whole project
+		// isn't C23-ready.
+		"-Wno-c23-extensions",
 		// Designated initializer syntax is recommended by the Google C++ style
 		// and is OK to use even if not formally supported by the chosen C++
 		// version.