[gcc r17-3199] libstdc++: Fixed iosfwd/synopsis.cc test for _GLIBCXX_USE_CXX11_ABI=0.

[Tomasz Kaminski]

https://gcc.gnu.org/g:9cc1aa01059a1772cded7068b2571fc750505dc9

commit r17-3199-g9cc1aa01059a1772cded7068b2571fc750505dc9
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date:   Tue Aug 11 10:52:54 2026 +0200

    libstdc++: Fixed iosfwd/synopsis.cc test for _GLIBCXX_USE_CXX11_ABI=0.
    
    __cpp_lib_syncbuf (and the corresponding feature) is only available for
    CXX11 string API.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/27_io/headers/iosfwd/synopsis.cc: Test __cpp_lib_syncbuf
            value only if _GLIBCXX_USE_CXX11_ABI.

Diff:
---
 libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc b/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc
index 81b158b81285..ddbb64116191 100644
--- a/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc
+++ b/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc
@@ -11,7 +11,7 @@
 # error "Feature-test macro __cpp_lib_spanstrean has wrong value in <iosfwd>"
 #endif
 
-#if __cplusplus >= 202002L && __cpp_lib_syncbuf != 201803L
+#if __cplusplus >= 202002L && _GLIBCXX_USE_CXX11_ABI && __cpp_lib_syncbuf != 201803L
 # error "Feature-test macro __cpp_lib_syncbuf has wrong value in <iosfwd>"
 #endif