Import <stdalign.h>

This header conforms to the C11 standard
Reference: ISO/IEC 9899:201x 7.15 Alignment <stdalign.h>

According to ISO/IEC 9899:201x (draft) 7.15 Alignment <stdalign.h> defines
four macros:
- alignas which expands to _Alignas
- alignof which expands to _Alignof
- __alignas_is_defined and __alignof_is_defined which both expand to 1

The _Alignas declaration appears as one of the type specifiers to modify
the alignment requirement of the object being declared.

The _Alignof operator is used to query the alignment requirement of its
operand type.

ISO/IEC N3242=11-0012 (C++1x) and ISO/IEC N3797 (C++1y) both note a header
<cstdalign> which defines only __alignas_is_defined and shall not define
the alignas macro. It misses the alignof case as it's probably based on an
older C1x draft, which defined only alignas. Assume that this is a bug in
the standard and treat alignof the same way as alignas in C++11.

Allow to define alignas and alignof in C++ prior the C++11 standard. It
might be broken but a nonstandard C++ compiler might support C11-like
_Alignas and _Alignof. Note that it's fatal for g++(1) v.5.4.

(kamil)