This function compiles with the standard, so say so.

This function compiles with the standard, so say so.

Add a note about not touching errno and warn about previous drafts
of the standard which changed the level of indirection to the thread
argument. POSIX had a bit of trouble deciding what to do. So anyone
coding to both draft 4 and draft 10 (the final draft) will get burnt
by this function. I did. Grrr.