macOS Catalina, asdf 0.8.0 installed via homebrew.

Current Behaviour

Installing ruby 3.0.0 will not only fetch ruby-build, but also build [email protected]..
Unless before hand I have this set up.

export RUBY_CONFIGURE_OPTS="--with-openssl-dir=$(brew --prefix [email protected])"

and make sure this dir contains ruby-build.

ruby_build_dir() {
    echo "$(asdf_ruby_plugin_path)/ruby-build"
}

Expected behavior

It should lookup to see if ruby-build is available in path, maybe it's installed from Homebrew before fetching it.
It should also check for dependencies installed via various package managers instead of building from scratch.

I find the sane defaults used by ruby-install to lookup dependencies via the local package manager to be a better option than to require pre-setup before you can use this plugin.

ruby-build does not check for system dependencies before downloading and attempting to compile the Ruby source. Please ensure that all requisite libraries are available on your system.

I find this to be poor user experience, especially since it requires up front work and maintaining it yourself with changes.

PR in #205