More C modernization.

* Makefile (typecheck): Add -D__time_t_defined to CFLAGS,
for glibc 2.15.
* asctime.c (asctime_r, asctime):
* date.c (main, dogmt, reset, nondigit, sametm, convert, checkfinal):
* ialloc.c (icatalloc):
* localtime.c (detzcode, detzcode64, settzname, tzload)
(typesequiv, getzname, getqzname, getnum, getsecs, getoffset)
(getrule, transtime, tzparse, tzset, localsub, gmtsub)
(leaps_thru_end_of, timesub, increment_overflow)
(long_increment_overflow, normalize_overflow)
(long_normalize_overflow, tmcomp, time2sub, time1, leapcorr):
* scheck.c (scheck):
* strftime.c (_yconv):
* zdump.c (my_localtime, abbrok, main, yeartot, hunt)
(delta, show, abbr, dumptime):
* zic.c (main, dolink, itsdir, associate, infile, inrule)
(inzone, inzcont, inzsub, inleap, inlink, rulesub, convert64)
(writezone, doabbr, stringoffset, stringrule, stringzone)
(outzone, addtype, leapadd, adjleap, ciequal, itsabbr, byword)
(getfields, rpytime, newabbr, mkdirs):
Omit 'register', as it adds no value these days.  See Christos Zoulas in
<http://mm.icann.org/pipermail/tz/2012-October/018376.html>.
* date.c (netsettime) [TSP_SETDATE]: Add forward decl.
Don't rely on implicit int.  Now static.  Use NULL, not 0.
* zdump.c: Include "limits.h".
(absolute_min_time, absolute_max_time): Compute at compile-time,
as the run-time tests relied on undefined behavior.
(checkabsolutes): Rename from setabsolutes.  Just check, since
there's nothing to set now.  All uses changed.

Makefile

@@ -455,7 +455,7 @@ typecheck:
 		make clean
 		for i in "long long" unsigned double; \
 		do \
-			make CFLAGS="-DTYPECHECK -D_TIME_T \"-Dtime_t=$$i\"" ; \
+			make CFLAGS="-DTYPECHECK -D__time_t_defined -D_TIME_T \"-Dtime_t=$$i\"" ; \
 			./zdump -v Europe/Rome ; \
 			make clean ; \
 		done