Make _Static_assert() work with GCC in older C++ standards.

GCC only activates C11 keywords in C mode, not C++ mode. This means
that when targeting an older C++ standard, we cannot fall back to using
_Static_assert(). In this case, do define _Static_assert() as a macro
that uses a typedef'ed array.

Discussed in: r322875 commit thread
Reported by: Mark MIllard
MFC after: 1 month