Simplify configuration of TM_GMTOFF and TM_ZONE.

* private.h, zdump.c (_GNU_SOURCE): New macro.
(TM_GMTOFF) [!NO_TM_GMTOFF]:
(TM_ZONE) [!NO_TM_ZONE]: Guess definition if not already defined.
* Makefile, NEWS: Document the above.

Makefile

@@ -161,25 +161,12 @@ GCC_DEBUG_FLAGS = -Dlint -g3 -O3 -fno-common -fstrict-aliasing \
 # (or if you decide to add such a field in your system's "time.h" file),
 # add the name to a define such as
 #	-DTM_GMTOFF=tm_gmtoff
-# or
-#	-DTM_GMTOFF=_tm_gmtoff
-# to the end of the "CFLAGS=" line.
-# Neither tm_gmtoff nor _tm_gmtoff is described in X3J11's work;
-# in its work, use of "tm_gmtoff" is described as non-conforming.
-# Both Linux and BSD have done the equivalent of defining TM_GMTOFF in
-# their recent releases.
-#
-# If your system has a "zone abbreviation" field in its "struct tm"s
-# (or if you decide to add such a field in your system's "time.h" file),
-# add the name to a define such as
+# to the end of the "CFLAGS=" line.  If not defined, the code attempts to
+# guess TM_GMTOFF from other macros; define NO_TM_GMTOFF to suppress this.
+# Similarly, if your system has a "zone abbreviation" field, define
 #	-DTM_ZONE=tm_zone
-# or
-#	-DTM_ZONE=_tm_zone
-# to the end of the "CFLAGS=" line.
-# Neither tm_zone nor _tm_zone is described in X3J11's work;
-# in its work, use of "tm_zone" is described as non-conforming.
-# Both UCB and Sun have done the equivalent of defining TM_ZONE in
-# their recent releases.
+# and define NO_TM_ZONE to suppress any guessing.  These two fields are not
+# required by POSIX, but are widely available on GNU/Linux and BSD systems.
 #
 # If you want functions that were inspired by early versions of X3J11's work,
 # add

NEWS

@@ -51,6 +51,10 @@ Unreleased, experimental changes
     Although not needed for tz's own applications, which are single-threaded,
     this supports POSIX better if the tz library is used in multithreaded apps.
 
+    The tz code now attempts to infer TM_GMTOFF and TM_ZONE if not
+    already defined, to make it easier to configure on common platforms.
+    Define NO_TM_GMTOFF and NO_TM_ZONE to suppress this.
+
     tzselect -c now uses a hybrid distance measure that works better
     in Africa.  (Thanks to Alan Barrett for noting the problem.)
 

private.h

@@ -71,6 +71,9 @@
 #define ctime_r _incompatible_ctime_r
 #endif /* HAVE_INCOMPATIBLE_CTIME_R */
 
+/* Enable tm_gmtoff and tm_zone on GNUish systems.  */
+#define _GNU_SOURCE 1
+
 /*
 ** Nested includes
 */
@@ -337,6 +340,19 @@ time_t posix2time(time_t);
 # endif
 #endif
 
+/* Infer TM_ZONE on systems where this information is known, but suppress
+   guessing if NO_TM_ZONE is defined.  Similarly for TM_GMTOFF.  */
+#if (defined __GLIBC__ \
+     || defined __FreeBSD__ || defined __NetBSD__ || defined __OpenBSD__ \
+     || (defined __APPLE__ && defined __MACH__))
+# if !defined TM_GMTOFF && !defined NO_TM_GMTOFF
+#  define TM_GMTOFF tm_gmtoff
+# endif
+# if !defined TM_ZONE && !defined NO_TM_ZONE
+#  define TM_ZONE tm_zone
+# endif
+#endif
+
 /*
 ** Private function declarations.
 */

zdump.c

@@ -18,6 +18,9 @@
 # include "private.h"
 #endif
 
+/* Enable tm_gmtoff and tm_zone on GNUish systems.  */
+#define _GNU_SOURCE 1
+
 #include "stdio.h"	/* for stdout, stderr, perror */
 #include "string.h"	/* for strcpy */
 #include "sys/types.h"	/* for time_t */
@@ -73,6 +76,19 @@ typedef long intmax_t;
 # endif
 #endif
 
+/* Infer TM_ZONE on systems where this information is known, but suppress
+   guessing if NO_TM_ZONE is defined.  Similarly for TM_GMTOFF.  */
+#if (defined __GLIBC__ \
+     || defined __FreeBSD__ || defined __NetBSD__ || defined __OpenBSD__ \
+     || (defined __APPLE__ && defined __MACH__))
+# if !defined TM_GMTOFF && !defined NO_TM_GMTOFF
+#  define TM_GMTOFF tm_gmtoff
+# endif
+# if !defined TM_ZONE && !defined NO_TM_ZONE
+#  define TM_ZONE tm_zone
+# endif
+#endif
+
 
 #ifndef ZDUMP_LO_YEAR
 #define ZDUMP_LO_YEAR	(-500)