unlink/rpl_unlink problem with MSVC

[Michael Goffioul]

I found yet another gnulib replacement problem with MSVC.
This time with unlink. In liboctave, file_ops::unlink is correctly
defined and exported, the problem is when linking liboctinterp:
I get 2 undefined references to file_ops::rpl_unlink and Funlink.
Indeed when I check the symbols in syscalls.o, I see
Frpl_unlink instead of Funlink.

Examining where it comes from, it looks to come from the
gnulib version of unistd.h.

Michael.