C++11 has a
  static_assert (COND, MESSAGE)
which gives more readable error messages for STATIC_ASSERT than our
current implementation.

This patch makes us use it if __cplusplus >= 201103L

There's also a provisional static_assert (COND) in C++1z, but presumably
we should wait until that one is fully standardized before using it.

gcc/ChangeLog:
	* system.h (STATIC_ASSERT): Use static_assert if building
	with C++11 onwards.

From-SVN: r238786