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, 1 insertions, 2 deletions

diff --git a/missing/file.h b/missing/file.h
index 241d716563..5e25c4761c 100644
--- a/missing/file.h
+++ b/missing/file.h
@@ -1,7 +1,6 @@
-/* This is file FILE.H */
-
 #ifndef _FILE_H_
 #define _FILE_H_
+/* This is file FILE.H */
 
 #include <fcntl.h>