Gem::Installer#build_extensions

# File tmp/rubies/ruby-4.1.0-preview1/lib/rubygems/installer.rb, line 838
def build_extensions
  if options[:build_extension] == false
    warn_skipped_extensions
    return
  end

  builder = Gem::Ext::Builder.new spec, build_args, Gem.target_rbconfig, build_jobs

  builder.build_extensions
end