Debian build: Make build reproducible

Summary: Diff taken from https://bugs.debian.org/783290 for testing purposes.

Test Plan: The idea itself is acceptable, of course, but, importantly, needs to support building without specified build-timestamp (need to adjust rkpackage_install.cmake.in, only, I think).

Subscribers: RKWard-Devel-list, tfry

Differential Revision: https://phabricator.kde.org/D104