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.