[ruby-core:103323] [Ruby master Bug#17788] ruby 3.0.0 - compiling c++ extension on alpine fails

Issue #17788 has been reported by memel (Maia Engeli).

----------------------------------------
Bug #17788: ruby 3.0.0 - compiling c++ extension on alpine fails
https://bugs.ruby-lang.org/issues/17788

* Author: memel (Maia Engeli)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-linux-musl]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
Compiling gem with C++ extension fails on alpine 3xx with Ruby 3.0.0 and Ruby 3.0.1 
(does not fail on Ubuntu 18 ¯\\_(ツ)_/¯)

Error:

```
compiling testgem.cc
In file included from /root/.rbenv/versions/3.0.0/include/ruby-3.0.0/ruby/ruby.h:39,
                 from /root/.rbenv/versions/3.0.0/include/ruby-3.0.0/ruby.h:38,
                 from testgem.cc:2:
/root/.rbenv/versions/3.0.0/include/ruby-3.0.0/ruby/internal/memory.h:275:16: error: conflicting declaration of 'void* ruby_nonempty_memcpy(void*, const void*, size_t)' with 'C' linkage
  275 | #define memcpy ruby_nonempty_memcpy
      |                ^~~~~~~~~~~~~~~~~~~~
/root/.rbenv/versions/3.0.0/include/ruby-3.0.0/ruby/internal/memory.h:265:1: note: previous declaration with 'C++' linkage
  265 | ruby_nonempty_memcpy(void *dest, const void *src, size_t n)
      | ^~~~~~~~~~~~~~~~~~~~
```

Code for reproduction:

extconf.rb
```
require 'mkmf'
create_makefile 'testgem'
```

testgem.cc
```
#include <stdio.h>
#include <ruby.h>
#include <iostream>

VALUE says(VALUE _self, VALUE str) {
    Check_Type(str, T_STRING);
    puts(StringValueCStr(str));
    return Qnil;
}

void Init_testgem() {
    VALUE mod = rb_define_module("Simon");
    const int num_args = 1;
    std::cout << "*** testgem initialized ***" << std::endl;
    rb_define_module_function(mod, "says", says, num_args);
}

---Files--------------------------------
testgem.cc (394 Bytes)
extconf.rb (41 Bytes)


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