Tux Paint Config. Merge Request #2: Allow to override build date (rejected)

patch comes from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=834780

Allow to override build date
in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

This date call works with GNU and BSD variants of date.

[Bernhard M. Wiedemann]

just found that this was already merged with changelog added in

commit 0e71c35d59c795de571d61c00589295da067a840
Author: Bill Kendrick <bill@smashwords.com>
Date:   Wed Apr 11 23:36:13 2018 -0700

+  * Building:
+    * Allow to override build date (in order to make builds reproducible)
+      by Chris Lamb, pushed by Bernhard M. Wiedemann