Index: include/stddef.h
===================================================================
--- include/stddef.h
+++ include/stddef.h
@@ -63,10 +63,11 @@
 #endif
 
 #if __STDC_VERSION__ >= 201112L || __cplusplus >= 201103L
-#ifndef __CLANG_MAX_ALIGN_T_DEFINED
+#ifndef _MAX_ALIGN_T_DECLARED
 typedef	__max_align_t	max_align_t;
 #define __CLANG_MAX_ALIGN_T_DEFINED
 #define _GCC_MAX_ALIGN_T
+#define _MAX_ALIGN_T_DECLARED
 #endif
 #endif
 
Index: sys/sys/stddef.h
===================================================================
--- sys/sys/stddef.h
+++ sys/sys/stddef.h
@@ -38,6 +38,11 @@
 #define	_PTRDIFF_T_DECLARED
 #endif
 
+#ifndef _MAX_ALIGN_T_DECLARED
+typedef	__max_align_t	max_align_t;
+#define	_MAX_ALIGN_T_DECLARED
+#endif
+
 #define	offsetof(type, field)	__offsetof(type, field)
 
 #endif /* !_SYS_STDDEF_H_ */