Implementing SINGLE construct

A block like

#pragma omp single
{
  body;
}

becomes

if (GOMP_single_start ())
  body;
GOMP_barrier ();

while

#pragma omp single copyprivate(x)
  body;

becomes

datap = GOMP_single_copy_start ();
if (datap == NULL)
  {
    body;
    data.x = x;
    GOMP_single_copy_end (&data);
  }
else
  x = datap->x;
GOMP_barrier ();