pcre2grep uses fseek/ftell to handle file offsets.

This should switch to fseeko/ftello on POSIX systems (_fseeki64 on Windows). Even on 32-bit CPUs, files on disk can be larger than 4GiB, so size_t arithmetic is not wide enough.

There is some Autoconf and CMake mess to navigate around this (a wide variety of OS-specific defines to make off_t use its 64-bit definition).

Variables used to track buffer positions can remain as size_t, however, some narrowing casts to int could be removed.