Get SOURCE_EPOCH from the latest git tag instead of NEWS

Currently, each change to NEWS triggers a meson reconfigure that
changes SOURCE_EPOCH which causes a full rebuild. Since NEWS changes
relatively often, we have a full rebuild each time we pull from
master even if we pull semi-regularly. This is further compounded
when using branches since NEWS has a relatively high chance to
differ between branches which causes git to update the modification
time, leading to a full rebuild when switching between branches.

We fix this by using the creation time of the latest git tag instead.