Detecting large-file support in configure

I'm reviewing this patch
http://archives.postgresql.org/pgsql-patches/2007-04/msg00531.php
which contains this configure.in code to decide if it's safe to allow
use of non-segmented files:

if test $ac_cv_func_fseeko = yes; then
AC_SYS_LARGEFILE
fi

+ if test "$ac_cv_sys_large_files" = "no" -o "$segmented_files" = "yes"; then 
+ AC_DEFINE([USE_SEGMENTED_FILES], 1, 
+            [Define to split data file in 1GB segments.]) 
+ fi

AFAICT this is simply wrong. ac_cv_sys_large_files isn't set to reflect
whether the platform has largefile support or not; it's set to reflect
whether you need to #define _LARGE_FILES to get that. On a platform
where largefile support is native, or is instead enabled by setting
_FILE_OFFSET_BITS to 64, this test would refuse to allow
USE_SEGMENTED_FILES to be disabled.

Another problem is that it'll do the wrong thing if fseeko isn't
present, because then AC_SYS_LARGEFILE isn't executed at all.

A quick look at the configure script suggests that the available macros
don't really set anything that specifically tells you if they found
working largefile support. I'm considering doing AC_CHECK_SIZEOF(off_t)
and then looking at the result, but I wonder if anyone knows the
"approved" way to accomplish this.

regards, tom lane