gnulib itself has a replacement for <unistd.h> which redefines pipe2
as rpl_pipe2 (etc), which is why the apparently recursive call in the
implementation of pipe2 isn't actually recursive. Since I didn't copy
that file, none of that worked and instead on platforms which have
pipe2 it recursed.
Reported-by: Laszlo Ersek
Fixes: commit 908e41e556
5e1f158c40