Use GCC-style attribute syntax for Clang compatibility

diff -urN a/config.h.in b/config.h.in
--- a/config.h.in	2025-07-30 08:24:41.000000000 +0000
+++ b/config.h.in	2025-12-14 22:16:57.591317448 +0000
@@ -1146,11 +1146,11 @@
 # ifndef _GL_BRACKET_BEFORE_ATTRIBUTE
 #  if defined __clang__ && defined __cplusplus
 #   if !defined __apple_build_version__ && __clang_major__ >= 10
-#    define _GL_ATTRIBUTE_MAYBE_UNUSED [[__maybe_unused__]]
+#    define _GL_ATTRIBUTE_MAYBE_UNUSED __attribute__((__unused__))
 #   endif
 #  elif _GL_HAVE___HAS_C_ATTRIBUTE
 #   if __has_c_attribute (__maybe_unused__)
-#    define _GL_ATTRIBUTE_MAYBE_UNUSED [[__maybe_unused__]]
+#    define _GL_ATTRIBUTE_MAYBE_UNUSED __attribute__((__unused__))
 #   endif
 #  endif
 # endif