Index: head/include/stddef.h
===================================================================
--- head/include/stddef.h
+++ head/include/stddef.h
@@ -62,6 +62,14 @@
 #endif
 #endif
 
+#if __STDC_VERSION__ >= 201112L || __cplusplus >= 201103L
+#ifndef __CLANG_MAX_ALIGN_T_DEFINED
+typedef	__max_align_t	max_align_t;
+#define __CLANG_MAX_ALIGN_T_DEFINED
+#define __GCC_MAX_ALIGN_T
+#endif
+#endif
+
 #define	offsetof(type, member)	__offsetof(type, member)
 
 #endif /* _STDDEF_H_ */
Index: head/sys/sys/_types.h
===================================================================
--- head/sys/sys/_types.h
+++ head/sys/sys/_types.h
@@ -100,6 +100,11 @@
 #define	_CHAR32_T_DECLARED
 #endif
 
+typedef struct {
+	long long __max_align1 __aligned(_Alignof(long long));
+	long double __max_align2 __aligned(_Alignof(long double));
+} __max_align_t;
+
 typedef	__uint32_t	__dev_t;	/* device number */
 
 typedef	__uint32_t	__fixpt_t;	/* fixed point number */