see: http://permalink.gmane.org/gmane.emacs.xemacs.patches/14885

--

diff -urpN xemacs-21.5.34.orig/src/lisp.h xemacs-21.5.34/src/lisp.h
--- xemacs-21.5.34.orig/src/lisp.h	2013-08-21 10:43:44.000000000 -0700
+++ xemacs-21.5.34/src/lisp.h	2015-09-20 18:45:24.239887146 -0700
@@ -1154,6 +1154,8 @@ typedef int Boolint;
 
 /* ------------------------ alignment definitions ------------------- */
 
+#if (defined (__STDC_VERSION__) && __STDC_VERSION__ < 201112L) || \
+    (defined (__cplusplus) && __cplusplus < 201103L)
 /* No type has a greater alignment requirement than max_align_t.
    (except perhaps for types we don't use, like long double) */
 typedef union
@@ -1163,6 +1165,7 @@ typedef union
   struct { void (*f)(void); } f;
   struct { double d; } d;
 } max_align_t;
+#endif
 
 /* ALIGNOF returns the required alignment of a type -- i.e. a value such
    that data of this type must begin at a memory address which is a
@@ -1170,7 +1173,11 @@ typedef union
    as the type itself. */
 
 #ifndef ALIGNOF
-# if defined (__GNUC__) && (__GNUC__ >= 2)
+# if defined (__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+#   define ALIGNOF(type) _Alignof(type)
+# elif defined (__cplusplus) && __cplusplus >= 201103L
+#  define ALIGNOF(type) alignof(type)
+# elif defined (__GNUC__) && (__GNUC__ >= 2)
 /* gcc has an extension that gives us exactly what we want. */
 #  define ALIGNOF(type) __alignof__ (type)
 # elif ! defined (__cplusplus)