stddef: Fix a compilation error on AIX 7.2 with xlclang

[Bruno Haible]

On AIX 7.2 with xlclang, I see this compilation error:

xlclang -q64 -qthreaded -qtls -std=gnu11 -DHAVE_CONFIG_H -I. -I../../gltests 
-I..  -DGNULIB_STRICT_CHECKING=1  -DIN_GNULIB_TESTS=1  -I. -I../../gltests  
-I.. -I../../gltests/..  -I../gllib -I../../gltests/../gllib  
-I/home/haible/prefix64/include  -Wno-error -g -O2 -MT test-alignasof.o -MD -MP 
-MF .deps/test-alignasof.Tpo -c -o test-alignasof.o 
../../gltests/test-alignasof.c
In file included from ../../gltests/test-alignasof.c:21:
In file included from ../gllib/stddef.h:79:
In file included from /opt/IBM/xlC/16.1.0/include2/stddef.h:118:
/opt/IBM/xlC/16.1.0/include2/__stddef_max_align_t.h:40:3: error: typedef 
redefinition with different types ('struct max_align_t' vs 'long')
} max_align_t;
  ^
../gllib/stddef.h:70:14: note: previous definition is here
typedef long max_align_t;
             ^
1 error generated.

The reason is that after gnulib's definition of max_align_t, the xlc header 
files attempt to define max_align_t too. This is done in a separate header file 
__stddef_max_align_t.h, which is guarded by a '#ifndef 
__CLANG_MAX_ALIGN_T_DEFINED'.

This patch makes things work.


2023-03-16  Bruno Haible  <bruno@clisp.org>

        stddef: Fix a compilation error on AIX 7.2 with xlclang.
        * lib/stddef.in.h (__CLANG_MAX_ALIGN_T_DEFINED): Define whenever we
        define GNULIB_defined_max_align_t.

diff --git a/lib/stddef.in.h b/lib/stddef.in.h
index 9e9d4b7cc6..c7ce608b98 100644
--- a/lib/stddef.in.h
+++ b/lib/stddef.in.h
@@ -69,6 +69,7 @@ typedef long rpl_max_align_t;
 typedef long max_align_t;
 #     define _MAX_ALIGN_T
 #    endif
+#    define __CLANG_MAX_ALIGN_T_DEFINED
 #    define GNULIB_defined_max_align_t 1
 #   endif
 #  endif
@@ -137,6 +138,7 @@ typedef union
   long int __i _GL_STDDEF_ALIGNAS (long int);
 } rpl_max_align_t;
 #   define max_align_t rpl_max_align_t
+#   define __CLANG_MAX_ALIGN_T_DEFINED
 #   define GNULIB_defined_max_align_t 1
 #  endif
 # endif