Resolved Cannot find the declaration of constant: CONFIG

```
_gem/lib/go_gem/mkmf.rb:29:14: [warning] Cannot find the declaration of constant: `CONFIG`
│ Diagnostic ID: Ruby::UnknownConstant
│
└       case `#{CONFIG["CC"]} --version` # rubocop:disable Lint/LiteralAsCondition
                ~~~~~~
```

Author: sue445

_gem/lib/go_gem/mkmf.rb

@@ -26,7 +26,7 @@ def $objs.empty?; false; end
 
       create_makefile(target, srcprefix)
 
-      case `#{CONFIG["CC"]} --version` # rubocop:disable Lint/LiteralAsCondition
+      case `#{RbConfig::CONFIG["CC"]} --version` # rubocop:disable Lint/LiteralAsCondition
       when /Free Software Foundation/
         ldflags = "-Wl,--unresolved-symbols=ignore-all"
       when /clang/