Allow to override build date with SOURCE_DATE_EPOCH
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.
Also use UTC to be independent of timezone.
This fixes a regression of https://bugzilla.gnome.org/show_bug.cgi?id=789382 introduced with the switch from autoconf to meson.
An alternative solution would be to drop the date and reduce code.
This PR was done while working on reproducible builds for openSUSE.