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.h (HAVE_RUBY_THREAD_H): to show ruby/thread.h to be
available. fixup of r36355.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36359 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
extconf.rb do not need to check if headers exist under separated
directory. [ruby-dev:35437]
* include/{ruby,rubyio,rubysig}.h, include/ruby/intern.h: use
"ruby/..." instead of <ruby/...>.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@18039 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
