continuation.c: quiet static-linked-ext

* ext/continuation/continuation.c (Init_continuation): start
  quietly if statically linked.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48622 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/continuation/continuation.c

@@ -6,6 +6,8 @@ void ruby_Init_Continuation_body(void);
 void
 Init_continuation(void)
 {
+#ifndef RUBY_EXPORT
     rb_warn("callcc is obsolete; use Fiber instead");
+#endif
     ruby_Init_Continuation_body();
 }