[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>