helpers: don't override C++11's built-in static_assert

The #ifndef doesn't work with the built-in static_assert from C++11, so
it ends up getting redefined. It seems that there are other versions of
static_assert internally in some C++ standard library implementations
that have additional arguments, and these also get overwritten by the
two-argument version here.

Detect if we're using a suitably-recent version of C++ and avoid the
redefintion in that case.

Signed-off-by: Ray Smith <rsmith@brightsign.biz>