add #include guard hack

According to MSVC manual (*1), cl.exe can skip including a header file
when that:

- contains #pragma once, or
- starts with #ifndef, or
- starts with #if ! defined.

GCC has a similar trick (*2), but it acts more stricter (e. g. there
must be _no tokens_ outside of #ifndef...#endif).

Sun C lacked #pragma once for a looong time.  Oracle Developer Studio
12.5 finally implemented it, but we cannot assume such recent version.

This changeset modifies header files so that each of them include
strictly one #ifndef...#endif.  I believe this is the most portable way
to trigger compiler optimizations. [Bug #16770]

*1: https://docs.microsoft.com/en-us/cpp/preprocessor/once
*2: https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html

internal/enumerator.h

@@ -1,4 +1,6 @@
-/**                                                         \noop-*-C-*-vi:ft=c
+#ifndef INTERNAL_ENUMERATOR_H                            /*-*-C-*-vi:se ft=c:*/
+#define INTERNAL_ENUMERATOR_H
+/**
  * @file
  * @author     Ruby developers <ruby-core@ruby-lang.org>
  * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
@@ -7,8 +9,6 @@
  *             file COPYING are met.  Consult the file for details.
  * @brief      Internal header for Enumerator.
  */
-#ifndef INTERNAL_ENUMERATOR_H
-#define INTERNAL_ENUMERATOR_H
 #include "ruby/ruby.h"          /* for VALUE */
 #include "ruby/intern.h"        /* for rb_enumerator_size_func */