stddef: document the AIX xlc issue

[Bruno Haible]

On AIX with xlc in 64-bit mode, I'm seeing this configure test result:
  checking whether NULL can be used in arbitrary expressions... no

The cause is that sizeof NULL < sizeof (void *). Let's document it.


2019-12-12  Bruno Haible  <address@hidden>

        stddef: Document the AIX xlc issue.
        * doc/posix-headers/stddef.texi: Document the NULL issue with AIX xlc.

diff --git a/doc/posix-headers/stddef.texi b/doc/posix-headers/stddef.texi
index 05994d2..7dee0e7 100644
--- a/doc/posix-headers/stddef.texi
+++ b/doc/posix-headers/stddef.texi
@@ -22,6 +22,11 @@ Some old platforms fail to provide @code{wchar_t}.
 Some platforms provide a @code{NULL} macro that cannot be used in arbitrary
 expressions:
 NetBSD 5.0
+
+@item
+Some platforms provide a @code{NULL} macro whose value does not have the size
+of a pointer:
+AIX 7.2 with xlc in 64-bit mode.
 @end itemize
 
 Portability problems not fixed by Gnulib: