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 was formed by installing tzdata on a system with zic, and
zipping up the results, after adding a file VERSION giving the
version.  It should be possible to build the compiled databases on
Windows, but zic.c assumes links and so would need work.  (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.)

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=2015a

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

make -f Make.zi VERSION=2015a 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 does not
to suffice on OS X, whose zic seems to be 32-bit.