lseek: Fix the override to not undo the effects of AC_SYS_LARGEFILE. Reported by John Donoghue <john.david.donoghue@gmail.com> in <https://lists.gnu.org/archive/html/bug-gnulib/2020-01/msg00146.html>. * lib/lseek.c (rpl_lseek): When AC_SYS_LARGEFILE has enabled a 64-bit off_t on mingw, invoke _lseeki64 instead of lseek.