stdalign: Document a clang 17 bug

[Bruno Haible]

The configure test
  checking for alignas and alignof...
reports 'yes, <stdalign.h> macros' with gcc, but 'no' with
clang 17 and CC="clang -std=gnu2x". In my opinion, this is a clang bug.
This patch documents it.


2024-02-12  Bruno Haible  <bruno@clisp.org>

        stdalign: Document a clang 17 bug.
        * doc/posix-headers/stdalign.texi: Mention a clang 17 bug.

diff --git a/doc/posix-headers/stdalign.texi b/doc/posix-headers/stdalign.texi
index 7f75a15540..4f9c2dd186 100644
--- a/doc/posix-headers/stdalign.texi
+++ b/doc/posix-headers/stdalign.texi
@@ -45,6 +45,10 @@
 @code{alignasof} module, as such code should use @code{alignas} and
 @code{alignof} without checking these two macros.  (Gnulib's
 now-deprecated @code{stdalign} module defines these two macros.)
+@item
+@c https://github.com/llvm/llvm-project/issues/81472
+Clang 17 does not support an @code{alignas} attribute in the
+specifier-qualifier position of a member declaration.
 @end itemize
 
 Portability problems not fixed by Gnulib: