[ChangeLog][qmake] Allows to override _DATE_ with SOURCE_DATE_EPOCH 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. Fixes: QTBUG-115737 Change-Id: I1964aa0a3d849628440618aa6f0a51af2017d824 Reviewed-by: Alexey Edelev <alexey.edelev@qt.io> Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io> Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>