[ruby/date] Ignore warned variables

To suppress warnings at the compilation time.

https://github.com/ruby/date/commit/ff21132203

ext/date/extconf.rb

@@ -3,7 +3,9 @@ require 'mkmf'
 
 config_string("strict_warnflags") {|w| $warnflags += " #{w}"}
 
-have_var("timezone", "time.h")
-have_var("altzone", "time.h")
+with_werror("", {:werror => true}) do |opt, |
+  have_var("timezone", "time.h", opt)
+  have_var("altzone", "time.h", opt)
+end
 
 create_makefile('date_core')