libstdc++: Explicitly pass -Wsystem-headers in tests that need it

When running libstdc++ tests using an installed gcc (as opposed to an
in-tree gcc), we naturally use system stdlib headers instead of the
in-tree headers.  But warnings from within system headers are suppressed
by default, so tests that check for such warnings spuriously fail in such
a setup.  This patch makes us compile such tests with -Wsystem-headers so
that they consistently pass.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/bind/dangling_ref.cc: Compile with
	-Wsystem-headers.
	* testsuite/20_util/ratio/operations/ops_overflow_neg.cc: Likewise.
	* testsuite/20_util/unique_ptr/lwg4148.cc: Likewise.
	* testsuite/29_atomics/atomic/operators/pointer_partial_void.cc:
	Likewise.
	* testsuite/30_threads/packaged_task/cons/dangling_ref.cc:
	Likewise.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
(cherry picked from commit e690b97761e18daccb4fff0151c97c1d0115b55f)
diff --git a/libstdc++-v3/testsuite/20_util/bind/dangling_ref.cc b/libstdc++-v3/testsuite/20_util/bind/dangling_ref.cc
index 17e7b21..32af0a2 100644
--- a/libstdc++-v3/testsuite/20_util/bind/dangling_ref.cc
+++ b/libstdc++-v3/testsuite/20_util/bind/dangling_ref.cc
@@ -1,4 +1,5 @@
 // { dg-do compile { target c++11 } }
+// { dg-additional-options "-Wsystem-headers" }
 #include <functional>
 
 int f();
diff --git a/libstdc++-v3/testsuite/20_util/ratio/operations/ops_overflow_neg.cc b/libstdc++-v3/testsuite/20_util/ratio/operations/ops_overflow_neg.cc
index 5eff8e3..368866a 100644
--- a/libstdc++-v3/testsuite/20_util/ratio/operations/ops_overflow_neg.cc
+++ b/libstdc++-v3/testsuite/20_util/ratio/operations/ops_overflow_neg.cc
@@ -1,6 +1,6 @@
 // { dg-do compile { target c++11 } }
 // { dg-require-cstdint "" }
-
+// { dg-additional-options "-Wsystem-headers" }
 // 2008-07-03 Chris Fairles <chris.fairles@gmail.com>
 
 // Copyright (C) 2008-2025 Free Software Foundation, Inc.
diff --git a/libstdc++-v3/testsuite/20_util/unique_ptr/lwg4148.cc b/libstdc++-v3/testsuite/20_util/unique_ptr/lwg4148.cc
index c70d7a6..f5697e4 100644
--- a/libstdc++-v3/testsuite/20_util/unique_ptr/lwg4148.cc
+++ b/libstdc++-v3/testsuite/20_util/unique_ptr/lwg4148.cc
@@ -1,4 +1,5 @@
 // { dg-do compile { target c++11 } }
+// { dg-additional-options "-Wsystem-headers" }
 
 // LWG 4148. unique_ptr::operator* should not allow dangling references
 
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/operators/pointer_partial_void.cc b/libstdc++-v3/testsuite/29_atomics/atomic/operators/pointer_partial_void.cc
index e959418a1..1e2f71e 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic/operators/pointer_partial_void.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/operators/pointer_partial_void.cc
@@ -1,6 +1,7 @@
 // { dg-do run { target { c++11_only || c++14_only } } }
 // { dg-require-atomic-builtins "" }
 // { dg-require-effective-target hosted }
+// { dg-additional-options "-Wsystem-headers" }
 
 // Copyright (C) 2012-2025 Free Software Foundation, Inc.
 //
diff --git a/libstdc++-v3/testsuite/30_threads/packaged_task/cons/dangling_ref.cc b/libstdc++-v3/testsuite/30_threads/packaged_task/cons/dangling_ref.cc
index 51c6ade..83411c7 100644
--- a/libstdc++-v3/testsuite/30_threads/packaged_task/cons/dangling_ref.cc
+++ b/libstdc++-v3/testsuite/30_threads/packaged_task/cons/dangling_ref.cc
@@ -1,4 +1,5 @@
 // { dg-do compile { target c++11 } }
+// { dg-additional-options "-Wsystem-headers" }
 #include <future>
 
 // C++20 [futures.task.members]