[gcc r15-3808] libstdc++: guard c++config pragmas

[Jason Merrill]

https://gcc.gnu.org/g:358db2e3ed4acf44282d1d9ebbc4a1a3b6e38d21

commit r15-3808-g358db2e3ed4acf44282d1d9ebbc4a1a3b6e38d21
Author: Jason Merrill <jason@redhat.com>
Date:   Sat Sep 21 13:30:31 2024 -0400

    libstdc++: guard c++config pragmas
    
    c++config needs to be compilable as C, in which mode we complain about the
    -Wc++ pragmas.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/c++config: Don't try to disable -Wc++??-extensions
            when compiling as C.

Diff:
---
 libstdc++-v3/include/bits/c++config | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index 66d03cfd0375..16c67b80e769 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -34,8 +34,10 @@
 
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wvariadic-macros"
+#if __cplusplus
 #pragma GCC diagnostic ignored "-Wc++11-extensions"
 #pragma GCC diagnostic ignored "-Wc++23-extensions" // bf16
+#endif
 
 // The major release number for the GCC release the C++ library belongs to.
 #define _GLIBCXX_RELEASE