[gcc r16-7978] libstdc++: Use specific feature test macro for #if in <string>

[Jonathan Wakely]

https://gcc.gnu.org/g:57f353300f4cf074e6b6ab719c5db0d74708e204

commit r16-7978-g57f353300f4cf074e6b6ab719c5db0d74708e204
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sat Mar 7 12:14:27 2026 +0000

    libstdc++: Use specific feature test macro for #if in <string>
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h: Check __glibcxx_to_string instead
            of __cplusplus.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/include/bits/basic_string.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 9bbe16507d00..202a911f9ef3 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -56,11 +56,10 @@
 # include <bits/ranges_util.h>          // ranges::subrange
 #endif
 
-#if __cplusplus > 202302L
+#if __glibcxx_to_string >= 202306L // C++ >= 26
 # include <charconv>
 #endif
 
-
 #if ! _GLIBCXX_USE_CXX11_ABI
 # include "cow_string.h"
 #else