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, 3 insertions, 4 deletions

diff --git a/include/ruby/missing.h b/include/ruby/missing.h
index 4a5db6007a..a2d82f0746 100644
--- a/include/ruby/missing.h
+++ b/include/ruby/missing.h
@@ -1,4 +1,6 @@
-/**                                                     \noop-*-C++-*-vi:ft=cpp
+#ifndef RUBY_MISSING_H                               /*-*-C++-*-vi:se ft=cpp:*/
+#define RUBY_MISSING_H 1
+/**
  * @file
  * @author     $Author$
  * @date       Sat May 11 23:46:03 JST 2002
@@ -8,9 +10,6 @@
  *             file COPYING are met.  Consult the file for details.
  * @brief      Prototype for *.c in ./missing, and for missing timeval struct.
  */
-#ifndef RUBY_MISSING_H
-#define RUBY_MISSING_H 1
-
 #include "ruby/3/config.h"
 
 #ifdef STDC_HEADERS