[PATCH cygport] Add more checks of SOURCE_DATE_EPOCH

[Jon Turney]

On 16/02/2024 12:29, Christian Franke via Cygwin-apps wrote:
> Fail if it is out of range. Warn if it lies in the future. Inform 
> whether it is set or set but not exported.

What is the valid range here?

Would it not make more sense to just re-export it if set? (so that 
commands like "SOURCE_DATE_EPOCH=something cygport foo" work as expected?)