Workaround rubygems $LOAD_PATH bug

Ref: #647
Ref: rubygems/rubygems#6490

Older rubygems are executing `extconf.rb` with a broken `$LOAD_PATH`
causing the `json` gem native extension to be loaded with the stdlib
version of the `.rb` files.

This fails with

```
json/common.rb:82:in `initialize': wrong number of arguments (given 1, expected 0) (ArgumentError)
```

Since this is just for `extconf.rb` we can probably just accept that
extra argument and ignore it.

The bug was fixed in rubygems 3.4.9 / 2023-03-20

ext/json/ext/generator/generator.c

@@ -963,6 +963,12 @@ static VALUE cState_generate(VALUE self, VALUE obj)
     return result;
 }
 
+static VALUE cState_initialize(int argc, VALUE *argv, VALUE self)
+{
+    rb_warn("The json gem extension was loaded with the stdlib ruby code. You should upgrade rubygems with `gem update --system`");
+    return self;
+}
+
 /*
  * call-seq: initialize_copy(orig)
  *
@@ -1408,6 +1414,9 @@ void Init_generator(void)
     cState = rb_define_class_under(mGenerator, "State", rb_cObject);
     rb_define_alloc_func(cState, cState_s_allocate);
     rb_define_singleton_method(cState, "from_state", cState_from_state_s, 1);
+    rb_define_method(cState, "initialize", cState_initialize, -1);
+    rb_define_alias(cState, "initialize", "initialize"); // avoid method redefinition warnings
+
     rb_define_method(cState, "initialize_copy", cState_init_copy, 1);
     rb_define_method(cState, "indent", cState_indent, 0);
     rb_define_method(cState, "indent=", cState_indent_set, 1);

test/json/json_generator_test.rb

@@ -261,19 +261,19 @@ def test_buffer_initial_length
   end
 
   def test_gc
-    if respond_to?(:assert_in_out_err) && !(RUBY_PLATFORM =~ /java/)
-      assert_in_out_err(%w[-rjson -Ilib -Iext], <<-EOS, [], [])
-        bignum_too_long_to_embed_as_string = 1234567890123456789012345
-        expect = bignum_too_long_to_embed_as_string.to_s
-        GC.stress = true
-
-        10.times do |i|
-          tmp = bignum_too_long_to_embed_as_string.to_json
-          raise "'\#{expect}' is expected, but '\#{tmp}'" unless tmp == expect
-        end
-      EOS
+    pid = fork do
+      bignum_too_long_to_embed_as_string = 1234567890123456789012345
+      expect = bignum_too_long_to_embed_as_string.to_s
+      GC.stress = true
+
+      10.times do |i|
+        tmp = bignum_too_long_to_embed_as_string.to_json
+        raise "#{expect}' is expected, but '#{tmp}'" unless tmp == expect
+      end
     end
-  end if GC.respond_to?(:stress=)
+    _, status = Process.waitpid2(pid)
+    assert_predicate status, :success?
+  end if GC.respond_to?(:stress=) && Process.respond_to?(:fork)
 
   def test_configure_using_configure_and_merge
     numbered_state = {