Suppress warnings by clang 21

https://bugs.ruby-lang.org/issues/21629

Author: nobu

ext/date/extconf.rb

@@ -3,6 +3,9 @@
 
 config_string("strict_warnflags") {|w| $warnflags += " #{w}"}
 
+unless try_compile("", "", werror: true)
+  append_cflags("-Wno-default-const-init-field-unsafe")
+end
 append_cflags("-Wno-compound-token-split-by-macro") if RUBY_VERSION < "2.7."
 have_func("rb_category_warn")
 with_werror("", {:werror => true}) do |opt, |