extconf.rb: strict_warnflags

* ext/date/extconf.rb: add strict warning flags same as ruby
  itself for C99 option.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52205 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/date/extconf.rb

@@ -1,2 +1,3 @@
 require 'mkmf'
+config_string("strict_warnflags") {|w| $warnflags += " #{w}"}
 create_makefile('date_core')