zdump: use tz library unless USE_LTZ is 0

This fixes some schizophrenia in the build, which linked zdump to
the tz library code but did not compile zdump with the tz library API.
* zdump.c (USE_LTZ): New macro.
Use it, not time_tz, to decide whether to include private.h.
* Makefile, NEWS: Document this.
* Makefile (CHECK_TIME_T_ALTERNATIVES): New macro.
(public): Use it.

Makefile

@@ -136,6 +136,8 @@ LDLIBS=
 #	DST transitions if the time zone files cannot be accessed
 #  -DUNINIT_TRAP=1 if reading uninitialized storage can cause problems
 #	other than simply getting garbage data
+#  -DUSE_LTZ=0 to build zdump with the system time zone library
+#	Also set TZDOBJS=zdump.o and CHECK_TIME_T_ALTERNATIVES= below.
 #  -DZIC_MAX_ABBR_LEN_WO_WARN=3
 #	(or some other number) to set the maximum time zone abbreviation length
 #	that zic will accept without a warning (the default is 6)
@@ -258,6 +260,10 @@ VALIDATE_ENV = \
   SP_CHARSET_FIXED=YES \
   SP_ENCODING=UTF-8
 
+# This expensive test requires USE_LTZ.
+# To suppress it, define this macro to be empty.
+CHECK_TIME_T_ALTERNATIVES = check_time_t_alternatives
+
 # SAFE_CHAR is a regular expression that matches a safe character.
 # Some parts of this distribution are limited to safe characters;
 # others can use any UTF-8 character.
@@ -488,7 +494,7 @@ maintainer-clean: clean
 names:
 		@echo $(ENCHILADA)
 
-public:		check check_public check_time_t_alternatives \
+public:		check check_public $(CHECK_TIME_T_ALTERNATIVES) \
 		tarballs signatures
 
 date.1.txt:	date.1

NEWS

@@ -67,6 +67,11 @@ Unreleased, experimental changes
     more likely to guess right for ambiguous time stamps near
     transitions where tm_isdst does not change.
 
+    zdump now builds with the tz library unless USE_LTZ is defined to 0,
+    This lets zdump use tz features even if the system library lacks them.
+    To build zdump with the system library, use 'make CFLAGS=-DUSE_LTZ=0
+    TZDOBJS=zdump.o CHECK_TIME_T_ALTERNATIVES='.
+
     tzselect -c now uses a hybrid distance measure that works better
     in Africa.  (Thanks to Alan Barrett for noting the problem.)
 

zdump.c

@@ -9,12 +9,14 @@
 ** This code has been made independent of the rest of the time
 ** conversion package to increase confidence in the verification it provides.
 ** You can use this code to help in verifying other implementations.
-**
-** However, include private.h when debugging, so that it overrides
-** time_t consistently with the rest of the package.
+** To do this, compile with -DUSE_LTZ=0 and link without the tz library.
 */
 
-#ifdef time_tz
+#ifndef USE_LTZ
+# define USE_LTZ 1
+#endif
+
+#if USE_LTZ
 # include "private.h"
 #endif