REPRODUCIBLE_BUILD

REPRODUCIBLE_BUILD

Take the 4-5 line Debian patch and turn it into six dozen lines of
new code.  The submitted patch introduces use of several C library
routines that aren't presently in use, so would need testing by all
functional or nearly-functional ports to verify that it wouldn't
break anything.  It also switched the formatted build date+time
from localtime to UTC.  This makes the code conditional so it can
be ignored by anybody and avoid the risk of breakage.  And a lot of
the increase in size is comments attempting to explain what the new
conditional is for:  when REPRODUCIBLE_BUILD is defined, makedefs
will use getenv("SOURCE_DATE_EPOCH") (whose value is an integer
representing seconds since 1-Jan-1970) instead of current date+time
when generating date.h.  The purpose is to be able to rebuild at a
later date and produce an identical program, which doesn't happen
when compile time gets incorporated into the binary.

I've added some sanity checking to try to make sure the getenv()
value obtained isn't bogus.  And the version string put into date.h
will be slightly different, allowing someone who sees date.h or 'v'
output to tell whether SOURCE_DATE_EPOCH was involved:  showing
"<port> NetHack <version> last revision <date>" instead of the
usual "... last build <date>".

To test, checkout a new branch for building, make any local edits
to unixconf.h and config.h, including enabling REPRODUCIBLE_BUILD,
git add+commit them, then use
  SOURCE_DATE_EPOCH=`git log -1 --pretty=%ct` make install
Other ports will need a bit more work to set up the environment,
but can still use git to track file dates and supply the latest.
Building with alternate configurations could be accomplished by
using tags instead of 'log -1' or by using distinct build branches
where nothing is commited/merged/rebased after completed build.

Unresolved issue:  BUILD_DATE, VERSION_ID, and COPYRIGHT_BANNER_C
contain formatted date+time but omit timezone.  SOURCE_DATE_EPOCH
is assumed to be UTC but the formatted values don't say so, so it
might appear to be incorrect when compared with local time.  We
definitely don't want to start mucking about with timezones within
nethack, so I think we just live with this.  It's not an issue for
default configruation where REPRODUCIBLE_BUILD is left disabled.