timegm(3) implementation for platforms that don't have it (AIX is an example)
This is forked off https://github.com/lzsiga/timegm, but with some changes:
- Written for C++14 or later, using constexprs where appropriate.
- Uses days_from_civil as a better day calculation without an arbitrary minimal date, allowing arbitrary year and date representation filling the entire int range.
- Added small randomized test suite comparing to the linux timegm.
- Added a const-taking variant that works as a constexpr function.
- Handle leap seconds on input.