Wrong platform check for Xinerama support

[Tor Lillqvist]

> if test "$_os" = "Linux" -o "$_os" = "FreeBSD" -o "$_os" = "MirBSD" -o "$_os" = "OpenBSD" #etc, etc;
>    UNIX_X11=true || POSIX_OS=true
> fi
>
> and then test UNIX_X11 (or POSIX_OS).

I doubt we need any separate flag for POSIXness, because basically
anything that isn't Windows is "POSIX", surely. Actually, POSIX or not
is a bit too wide a test; after all the non-desktop OSes (iOS and
Android) have most of the normal POSIX system calls, but one sure is
not supposed/allowed to for instance fork() in an app on them.

--tml