Use cmake TIMESTAMP function

Bernhard M. Wiedemann requested to merge wiedemann/digikam:date into master

This greatly simplifies code and allows to override build date with SOURCE_DATE_EPOCH in order to make builds reproducible. See for why this is good and for the definition of this variable.

We still use UTC to be independent of timezone.

Without this patch, /usr/lib64/ always varied between clean builds.

Related to commit 6cc8e8b0 CCBUGS: 237010

This PR was done while working on reproducible builds for openSUSE.

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