Define max_align_t (using __max_align_t) in <sys/stddef.h>, add a common _MAX_ALIGN_T_DECLARED guard to <stddef.h> and <sys/stddef.h>

clang/gcc (<= 4.2) usage:

_Alignas(_Alignof(max_align_t))	uint8_t ivars[];

clang/gcc (>= 4.7) usage:

_Alignas(max_align_t)		uint8_t ivars[];

• clang++ -std=c++11 -xc++ -c test.c
• clang -std=c11 -c test.c
• g++ -std=c++11 -xc++ -c test.c
• gcc -std=c11 -c test.c

test.c:

#ifdef __cplusplus
#include <cstddef>
#else
#include <stddef.h>
#endif

struct foo {
    	_Alignas(_Alignof(max_align_t)) unsigned char ivars[32];
};