[ruby/date] Suppress compound-token-split-by-macro warnings

It was used intentionally.

https://github.com/ruby/date/commit/291b40f939

ext/date/extconf.rb

@@ -3,6 +3,7 @@ require 'mkmf'
 
 config_string("strict_warnflags") {|w| $warnflags += " #{w}"}
 
+append_cflags("-Wno-compound-token-split-by-macro") if RUBY_VERSION < "2.7."
 have_func("rb_category_warn")
 with_werror("", {:werror => true}) do |opt, |
   have_var("timezone", "time.h", opt)