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, 5 insertions, 1 deletions

diff --git a/include/ruby/backward/2/stdarg.h b/include/ruby/backward/2/stdarg.h
index 7b37bd4571..2270e66b77 100644
--- a/include/ruby/backward/2/stdarg.h
+++ b/include/ruby/backward/2/stdarg.h
@@ -1,4 +1,6 @@
-/**                                                     \noop-*-C++-*-vi:ft=cpp
+#ifndef RUBY_BACKWARD2_STDARG_H                      /*-*-C++-*-vi:se ft=cpp:*/
+#define RUBY_BACKWARD2_STDARG_H
+/**
  * @file
  * @author     Ruby developers <ruby-core@ruby-lang.org>
  * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
@@ -41,3 +43,5 @@
 #else
 #define ANYARGS
 #endif
+
+#endif /* RUBY_BACKWARD2_STDARG_H */