mkmf.rb: werror of try_cppflags

* lib/mkmf.rb (try_cppflags): set werror flag as same as cflags
  and ldflags.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50216 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -609,8 +609,8 @@ MSG
     $CPPFLAGS = cppflags unless ret
   end
 
-  def try_cppflags(flags)
-    try_header(MAIN_DOES_NOTHING, flags)
+  def try_cppflags(flags, opts = {})
+    try_header(MAIN_DOES_NOTHING, flags, {:werror => true}.update(opts))
   end
 
   def with_cflags(flags)