Use NEWS date instead of build date

Approver's checklist:

• The author has submitted the FSF Copyright Assignment and is listed in AUTHORS
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with existing code
• This feature/change has adequate documentation added (if appropriate)
• No obvious mistakes / misspelling in the code

Use NEWS date instead of build date
to make builds reproducible.
See https://reproducible-builds.org/ for why this is good.

This also avoids claiming copyright for future years.

Without this patch, building today's wget2 in 2030 would claim Copyright 2015-2030 Free Software Foundation, Inc

Note: this date call works with GNU date and (some) BSD date - not Windows or Solaris date.