Allow to override build date with SOURCE_DATE_EPOCH

Bernhard M. Wiedemannrequested to merge
bmwiedemann/gtk:gtk-4-4 into gtk-4-4

Allow to override build date with SOURCE_DATE_EPOCH 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 only works with GNU date.

Also use UTC to be independent of timezone.

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