Fix hurd bootstrap after 4c6f92daea

Hurd bootstrap with build-many-glibc.py fails for i686-gnu since
4c6f92daea:

In file included from zic.c:16:
private.h:849:1: error: static declaration of ‘mempcpy’ follows non-static declaration
  849 | mempcpy(void *restrict s1, void const *restrict s2, size_t n)
      | ^~~~~~~
In file included from ../include/string.h:60,
                 from private.h:222:
../string/string.h:432:14: note: previous declaration of ‘mempcpy’ with type [...]
  432 | extern void *mempcpy (void *__restrict __dest,
      |              ^~~~~~~

The libc-symbols.h already defined some HAVE_*, but timezone files are
built with -D_ISOMAC.  Remove its usage and only define _ and N_
macros if not already defined.

Checked on x86_64-linux-gnu and with a build-many-glibcs.py build for
i686-gnu.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Paul Eggert <eggert@cs.ucla.edu>

include/libc-symbols.h

@@ -113,6 +113,7 @@
 #define HAVE_LIBINTL_H	1
 #define HAVE_WCTYPE_H	1
 #define HAVE_ISWCTYPE	1
+#define HAVE_MEMPCPY	1
 #define ENABLE_NLS	1
 
 /* The symbols in all the user (non-_) macros are C symbols.  */

timezone/Makefile

@@ -67,7 +67,7 @@ tz-cflags = -DTZDIR='"$(zonedir)"' \
 	    -DTZDEFAULT='"$(localtime-file)"' \
 	    -DTZDEFRULES='"$(posixrules-file)"' \
 	    -DTM_GMTOFF=tm_gmtoff -DTM_ZONE=tm_zone \
-	    -DHAVE_GETTEXT -DUSE_LTZ=0 -D_ISOMAC -DTZ_DOMAIN='"libc"' \
+	    -DHAVE_GETTEXT -DUSE_LTZ=0 -DTZ_DOMAIN='"libc"' \
 	    -include $(common-objpfx)config.h $(config-cflags-wno-maybe-uninitialized)
 
 CFLAGS-zdump.c += $(tz-cflags)

timezone/private.h

@@ -1071,12 +1071,16 @@ time_t timeoff(struct tm *, long);
 ** The default is to use gettext if available, and use MSGID otherwise.
 */
 
-#if HAVE_GETTEXT
-# define _(msgid) gettext(msgid)
-#else /* !HAVE_GETTEXT */
-# define _(msgid) (msgid)
-#endif /* !HAVE_GETTEXT */
-#define N_(msgid) (msgid)
+#ifndef _
+# if HAVE_GETTEXT
+#  define _(msgid) gettext(msgid)
+# else
+#  define _(msgid) (msgid)
+# endif
+#endif
+#ifndef N_
+# define N_(msgid) (msgid)
+#endif
 
 #if !defined TZ_DOMAIN && defined HAVE_GETTEXT
 # define TZ_DOMAIN "tz"