unistd.h first?

[Karl Berry]

I was under the impression that <unistd.h> should be included before
just about any other system header, because it's <unistd.h> that defines
_POSIX_VERSION, which can affect just about anything.

I noticed that in autoconf 2.59, unistd.h is actually included last in
the ac_includes_default fragment.

I don't have a specific case that fails, but I thought I'd mention it
anyway ...

FWIW,
k