WIP: Use git commit or ChangeLog date instead of build date

Add a description of the new feature/bug fix. Reference any relevant bugs.

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code

Without this patch, building gnutls in the year 2034 produced such a diff in various binaries:

/usr/bin/certtool differs in ELF section .rodata
@@ -1150,7 +1150,7 @@
  2e790 00000000 00000000 00000000 00000000  ................
  2e7a0 63657274 746f6f6c 20332e36 2e360a43  certtool 3.6.6.C
  2e7b0 6f707972 69676874 20284329 20323030  opyright (C) 200
- 2e7c0 302d3230 31392046 72656520 536f6674  0-2019 Free Soft
+ 2e7c0 302d3230 33342046 72656520 536f6674  0-2034 Free Soft
  2e7d0 77617265 20466f75 6e646174 696f6e2c  ware Foundation,
  2e7e0 20616e64 206f7468 6572732c 20616c6c   and others, all
  2e7f0 20726967 68747320 72657365 72766564   rights reserved

See https://reproducible-builds.org/ for why this matters.

Also claiming Copyright for future years seems wrong.