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

1 files changed, 9 insertions, 1 deletions

diff --git a/include/ruby/backward.h b/include/ruby/backward.h
index 04ce2b8c52..8380152db5 100644
--- a/include/ruby/backward.h
+++ b/include/ruby/backward.h
@@ -1,5 +1,13 @@
-#ifndef RUBY_RUBY_BACKWARD_H
+#ifndef RUBY_RUBY_BACKWARD_H                         /*-*-C++-*-vi:se ft=cpp:*/
 #define RUBY_RUBY_BACKWARD_H 1
+/**
+ * @file
+ * @author     Ruby developers <ruby-core@ruby-lang.org>
+ * @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.
+ */
 #include "ruby/3/core/rbasic.h"
 #include "ruby/3/value.h"
 #include "ruby/3/interpreter.h"