MRUBY_PACKAGE_DIR does not always have a value.

[pyama86]

Hello,

I have a concern regarding the changes below. It seems that some of the builds may not work with the following modification:

Link to the changes

Since MRUBY_PACKAGE_DIR does not always have a value, it appears that builds used by projects relying on libmruby.a might be affected. If possible, I would appreciate it if we could revert to using libmruby_static as it was done in versions prior to 3.2, instead of utilizing libmruby_static.replace_prefix_by(build_dir => "$(MRUBY_PACKAGE_DIR)").

[dearblue]

Sorry. It seems I made an incorrect confirmation the last time I did this.
I think I was able to fix it in #6069, but it would be helpful if you could check.

[pyama86]

Thank you for promptly creating the patch. I confirmed that it worked without any issues with the provided content. I appreciate your swift response.

[dearblue]

Thank you for confirming!