main: distribute leapseconds in tzdata, and fix timestamps

Problem with leapseconds reported by Matt Burgess in
<http://mm.icann.org/pipermail/tz/2013-September/020379.html>
with a followup by Ian Abbott in
<http://mm.icann.org/pipermail/tz/2013-September/020383.html>.
* Makefile (MANTXTS): New macro.
(maintainer-clean): Use it.
(DOCS): Add it.
(DATA): Add 'leapseconds'.
(LEAP_DEPS): New macro.
(leapseconds): Use it.
(public): Don't depend on set-timestamps; each tarball does that
individually now, to allow for more parallelization.
(date.1.txt, newctime.3.txt, newstrftime.3.txt, newtzset.3.txt)
(time2posix.3.txt, tzfile.5.txt, tzselect.8.txt, zdump.8.txt)
(zic.8.txt): Depend on the corresponding non-.txt file.
$(MANTXTS): New rule.
(set-timestamps.out): Rename from set-timestamps, and make it a file,
so that it has its own timestamp; this avoids useless rework.
Depend on $(ENCHILADA) and ignore files under Git that are not
also listed in $(ENCHILADA).  Set the timestamp of 'leapseconds'
and each $(MANTXTS) to be the maximum of its dependencies, to avoid
timestamp churn in future tarballs.  Prefer touch -m, as we don't
care about access time here.
(tzcode$(VERSION).tar.gz, tzdata$(VERSION).tar.gz):
Depend on set-timestamps.out rather than the individual files,
so timestamps are set before creating the tarballs even when
make is parallelized.
(tzcode$(VERSION).tar.gz): Don't mention *.[1-8].txt explicitly,
since we now get $(MANTXTS) via $(DOCS).
* NEWS: Document the above.