Add static_assert macro definition in <assert.h>

This declaration conforms to the C11 standard
Reference: ISO/IEC 9899:201x 7.2 Diagnostics <assert.h>

_Static_assert performs compile-time assertion checking.

According to ISO/IEC 9899:201x (draft) 7.2 Diagnostics <assert.h> defines
the static_assert macro which expands to _Static_assert. It's not
conditionalized by NDEBUG like the assert macro.

According to ISO/IEC N3242=11-0012 (C++1x) the <cassert> header shall
define only the assert macro, but not static_assert as it's already part
of the C++11 language.

Allow to define static_assert in C++ prior the C++11 standard. It might be
broken but a nonstandard C++ compiler might support C11-like _Static_assert
feature. Note that it's fatal for g++ 5.4, but it works for clang++ 3.8.1.

Approved by <joerg>.

(kamil)