Make the build reproducible

[sebastic]

@lamby provided a patch for the Debian package to make the build reproducible in Debian Bug #1012836:

Whilst working on the Reproducible Builds effort [0] we noticed that mapproxy could not be built reproducibly.

This is because the default value for the "last_change" field in the gpkg sqlite file contains the current date:

  last_change DATETIME NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ','now'))

Patch attached that uses SOURCE_DATE_EPOCH instead if it's available.

Other distributions benefit from having this change upstream.

[lamby]

Thanks! Was just about to do this PR myself. :)

[weskamm]

Thanks for your contribution!