to make the package build reproducible by default without
everyone having to discover the custom variable.
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 code assigns the plain integer to keep the code simple.
Otherwise we would have to deal with differences between GNU date
and BSD date or include extra build deps like perl or python.
Due to reversed conditionals, it wasn't being generated properly.
This also actually fixes its generation in librb by just copying the
Charybdis version. Unbundlers will want to take note of this.
we do this by removing the uname usage everywhere: it is not actually
used at runtime at all.
we keep the timestamp, because it is actually used in user_welcome()
but allow it to be overriden.
ideally, that timestamp would be completely removed, but I am not sure
what to put in its place, or if it would break some mysterious RFC (or
client!) if we remove that announcement.
