Use ftello() & fseeko() on any OS based on GNU libc

Use ftello() & fseeko() on any OS based on GNU libc

Strictly speaking, they are available in POSIX.1-2008 [1][2], so they
could be used on more platforms/OSes. To be cautious, enable them when
using GNU libc, since they have been available with that libc for a
very long time.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/ftell.html
[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/fseek.html