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

include/ruby/regex.h

@@ -1,16 +1,14 @@
-/**********************************************************************
-
-  regex.h -
-
-  $Author$
-
-  Copyright (C) 1993-2007 Yukihiro Matsumoto
-
-**********************************************************************/
-
-#ifndef ONIGURUMA_REGEX_H
+#ifndef ONIGURUMA_REGEX_H                            /*-*-C++-*-vi:se ft=cpp:*/
 #define ONIGURUMA_REGEX_H 1
-
+/**
+ * @file
+ * @author     $Author$
+ * @copyright  Copyright (C) 1993-2007 Yukihiro Matsumoto
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
 #if defined(__cplusplus)
 extern "C" {
 #if 0