[ruby-core:98610] [Ruby master Bug#16930] Excessive deprecation warnings for c++ compile units

Issue #16930 has been reported by anatolik (Anatol Pomozov).

----------------------------------------
Bug #16930: Excessive deprecation warnings for c++ compile units
https://bugs.ruby-lang.org/issues/16930

* Author: anatolik (Anatol Pomozov)
* Status: Open
* Priority: Normal
* ruby -v: 2.7.1
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
I am using Arch Linux with the latest Ruby 2.7.1.

I am compiling a simple C++ program like this

``` cpp
#include <ruby.h>

int main() {
    return 0;
}
```

And then compile as `g++ -c main.cxx -I/usr/include/ruby-2.7.0 -I/usr/include/ruby-2.7.0/x86_64-linux`.

I see a lot of warnings are printed to the user:

```
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:222:1: note: declared here
  222 | rb_rescue2(type *q, VALUE w, type *e, VALUE r, ...)
      | ^~~~~~~~~~
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp: In function ‘VALUE ruby::backward::cxxanyargs::rb_ensure(VALUE (*)(...), VALUE, VALUE (*)(...), VALUE)’:
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:251:14: warning: ‘VALUE ruby::backward::cxxanyargs::rb_ensure(VALUE (*)(...), VALUE, VALUE (*)(...), VALUE)’ is deprecated: Use of ANYARGS in this function is deprecated [-Wdeprecated-declarations]
  251 |     func1_t *t = reinterpret_cast<func1_t*>(q);
      |              ^
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:248:1: note: declared here
  248 | rb_ensure(type *q, VALUE w, type *e, VALUE r)
      | ^~~~~~~~~
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:251:42: warning: ‘VALUE ruby::backward::cxxanyargs::rb_ensure(VALUE (*)(...), VALUE, VALUE (*)(...), VALUE)’ is deprecated: Use of ANYARGS in this function is deprecated [-Wdeprecated-declarations]
  251 |     func1_t *t = reinterpret_cast<func1_t*>(q);
      |                                          ^
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:248:1: note: declared here
  248 | rb_ensure(type *q, VALUE w, type *e, VALUE r)
      | ^~~~~~~~~
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:252:14: warning: ‘VALUE ruby::backward::cxxanyargs::rb_ensure(VALUE (*)(...), VALUE, VALUE (*)(...), VALUE)’ is deprecated: Use of ANYARGS in this function is deprecated [-Wdeprecated-declarations]
  252 |     func1_t *y = reinterpret_cast<func1_t*>(e);
      |              ^
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:248:1: note: declared here
  248 | rb_ensure(type *q, VALUE w, type *e, VALUE r)
      | ^~~~~~~~~
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:252:42: warning: ‘VALUE ruby::backward::cxxanyargs::rb_ensure(VALUE (*)(...), VALUE, VALUE (*)(...), VALUE)’ is deprecated: Use of ANYARGS in this function is deprecated [-Wdeprecated-declarations]
  252 |     func1_t *y = reinterpret_cast<func1_t*>(e);
      |                                          ^
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:248:1: note: declared here
  248 | rb_ensure(type *q, VALUE w, type *e, VALUE r)
      | ^~~~~~~~~
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp: In function ‘VALUE ruby::backward::cxxanyargs::rb_thread_create(VALUE (*)(...), void*)’:
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:343:12: warning: ‘VALUE ruby::backward::cxxanyargs::rb_thread_create(VALUE (*)(...), void*)’ is deprecated: Use of ANYARGS in this function is deprecated [-Wdeprecated-declarations]
  343 |     ptr_t *e = reinterpret_cast<ptr_t*>(q);
      |            ^
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:340:1: note: declared here
  340 | rb_thread_create(type *q, void *w)
      | ^~~~~~~~~~~~~~~~
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:343:38: warning: ‘VALUE ruby::backward::cxxanyargs::rb_thread_create(VALUE (*)(...), void*)’ is deprecated: Use of ANYARGS in this function is deprecated [-Wdeprecated-declarations]
  343 |     ptr_t *e = reinterpret_cast<ptr_t*>(q);
      |                                      ^
/usr/include/ruby-2.7.0/ruby/backward/cxxanyargs.hpp:340:1: note: declared here
  340 | rb_thread_create(type *q, void *w)
      | ^~~~~~~~~~~~~~~~
```

These warnings are coming include/ruby/backward/cxxanyargs.hpp where 
 RUBY_CXX_DEPRECATED("Use of ANYARGS in this function is deprecated") are included across the file. These deprecation warnings are printed only because the backward compatibility file is included. But there seems no way to disable backward compatibility inclusion.

These warnings should be printed only when the deprecated code is used by the user. Otherwise the compiler should be silent.



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