Add an environment variable to always build openssl/libssl

[eregon]

• This is useful when one wants to avoid depending e.g. on which openssl version is installed in Homebrew, notably for https://github.com/ruby/setup-ruby/issues/668

[mislav]

For posterity: this adds an opt-out mechanism first requested in https://github.com/rbenv/ruby-build/pull/2275#issuecomment-1836367229

[eregon]

I documented it in the README, it should be good to go