Does not build with Clang

[NoRePercussions]

Describe the bug
OkapiLib does not build on Mac OS 13.1 / with Clang.

Steps To Reproduce
Compile from source with Clang on Mac OS 13.1. I cannot test other versions of Mac OS.

Expected behavior
std::chrono::high_resolution_clock behaves differently on different machines (in my case, it is an alias of steady_clock). to_time_t is not always a member of steady_clock, so calling on std::chrono::system_clock is a better option to get a time_t.

Some types of captures do not work with all compilers – the solution is to separate out explicit definitions of variable names and then use std::tie to assign values to them.

Versions
OkapiLib 5 (PurdueSigBots maintenance fork)