ext/date/extconf.rb: check for timezone and altzone

Instead of in configure.in. As of r28592 the HAVE_VAR_TIMEZONE and
HAVE_VAR_ALTZONE macros are only used by ext/date. ext/date doesn't care
whether they are int or long, so just check for the existence.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59939 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/date/date_core.c

@@ -7710,8 +7710,8 @@ datetime_s_now(int argc, VALUE *argv, VALUE klass)
 	s = 59;
 #ifdef HAVE_STRUCT_TM_TM_GMTOFF
     of = tm.tm_gmtoff;
-#elif defined(HAVE_VAR_TIMEZONE)
-#ifdef HAVE_VAR_ALTZONE
+#elif defined(HAVE_TIMEZONE)
+#ifdef HAVE_ALTZONE
     of = (long)-((tm.tm_isdst > 0) ? altzone : timezone);
 #else
     of = (long)-timezone;

ext/date/extconf.rb

@@ -1,4 +1,9 @@
 # frozen_string_literal: true
 require 'mkmf'
+
 config_string("strict_warnflags") {|w| $warnflags += " #{w}"}
+
+have_var("timezone", "time.h")
+have_var("altzone", "time.h")
+
 create_makefile('date_core')