Gem::Ext::CargoBuilder#maybe_resolve_ldflag_variable

# File tmp/rubies/ruby-3.4.1/lib/rubygems/ext/cargo_builder.rb, line 289
def maybe_resolve_ldflag_variable(input_arg, dest_dir, crate_name)
  var_matches = input_arg.match(/\$\((\w+)\)/)

  return input_arg unless var_matches

  var_name = var_matches[1]

  return input_arg if var_name.nil? || var_name.chomp.empty?

  case var_name
  # On windows, it is assumed that mkmf has setup an exports file for the
  # extension, so we have to create one ourselves.
  when "DEFFILE"
    write_deffile(dest_dir, crate_name)
  else
    RbConfig::CONFIG[var_name]
  end
end