localtime.c is a stripped-down version of that from tzcode, from
http://www.iana.org/time-zones, converted to use a 64-bit time_t on
all platforms.  Apart from removing unused code and merging in the
parts of private.h which are needed, the other main changes are

- to use environment variable TZDIR to point to the zoneinfo files, and
  failing that, R_HOME/share/zoneinfo.

- to call getTZinfo() to get the default timezone if TZ is unset.

strftime.c is a stripped-down version of that from tzcode, using the
system version for locale-dependent day/month/am/pm names.


zoneinfo.zip is formed by installing tzdata on a system with zic, and
zipping up the results, after adding a file VERSION giving the
version.  (The binary databases are byte streams, and so do not depend
on the architecture.  There are some comments about whether time_t is
signed, which it is on all sensible platforms.  This is only used with
R's implemntation, where time_t is int_fast64_t, hence signed.)

To remake it, download the current version of tzdataXXXXx.tar.gz from
http://www.iana.org/time-zones to this directory and run

make -f Make.zi VERSION=2018c

for the appropriate version.  If zic is not on the path, use something like

make -f Make.zi VERSION=2018c ZIC=/usr/sbin/zic

NB: it seems that this needs to be done on a system with 64-bit time_t
or the catalogs will be confined to 1902-2038.  And even that did not
suffice on macOS.  As a sanity check, the size of zoneinfo.zip should
be about 400KB if 64-bit: a 32-bit version will be about 280KB.