Sync extconf.rb with ruby/ruby

[peterzhu2118]

Adds commit ruby/ruby@6d8f396 and
ruby/ruby@c8b3bd4.

[nobu]

This is kept un-backported intentionally, so that these warnings will be fixed.

[nobu]

I changed my mind.
It seems tough to “fix” warnings, until we drop the support for older OpenSSLs.
However since the current way is too compiler dependent, how about this?
https://github.com/nobu/openssl/tree/openssl3-warnings

[peterzhu2118]

I think that looks good. But we should still sync the extconf.rb between the ruby/openssl and ruby/ruby right?

[nobu]

ruby/ruby@6d8f396 will be reverted and sync from here to ruby/ruby, in that case.

[hsbt]

What's the next action? I'm not sure we choose nobu@ee5347a instead of this pull-request.

[hsbt]

#576 is best way for this.