pgsql: Re-order configure tests to reflect the fact that the code

Log Message:
-----------
Re-order configure tests to reflect the fact that the code generated for
posix_fadvise and other file-related functions can depend on _LARGEFILE_SOURCE
and/or _FILE_OFFSET_BITS. Per report from Robert Treat.

Back-patch to 8.4. This has been wrong all along, but we weren't really using
posix_fadvise in anger before, and AC_FUNC_FSEEKO seems to mask the issue well
enough for that function.

Modified Files:
--------------
pgsql:
configure (r1.668 -> r1.669)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/configure?r1=1.668&r2=1.669)
configure.in (r1.620 -> r1.621)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/configure.in?r1=1.620&r2=1.621)