[Bug libstdc++/110574] --enable-cstdio=stdio_pure is incompatible with LFS

[keithp at keithp dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110574

--- Comment #5 from keithp at keithp dot com <keithp at keithp dot com> ---
Seems like using fseeko would be a reasonable choice here -- while it's not in
ISO C, it is in POSIX 2017.