Don't claim GCC predefines __POSIX__

[Joseph S. Myers]

On Sat, 18 Feb 2012, Alfred M. Szmidt wrote:

> It is a quick way to check though, so if a system is not POSIX
> (i.e. doesn't have <unistd.h>, and doesn't define _POSIX_VERSION) then
> it will fail.  If it works, then you should (in theory) have a POSIX

Yes, that gets back to my point of using an autoconf test (since "doesn't 
have <unistd.h>" isn't something to test in a #if directive).

I've now committed my patch.

-- 
Joseph S. Myers
joseph@codesourcery.com