rbinstall.rb: respect DESTDIR

* lib/rubygems.rb (Gem.install): pass optinos to
  Gem::DependencyInstaller.

* tool/rbinstall.rb (gem): respect DESTDIR.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46823 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems.rb

@@ -544,9 +544,9 @@ module Gem
   #   Fetching: minitest-3.0.1.gem (100%)
   #   => [#<Gem::Specification:0x1013b4528 @name="minitest", ...>]
 
-  def self.install name, version = Gem::Requirement.default
+  def self.install name, version = Gem::Requirement.default, **options
     require "rubygems/dependency_installer"
-    inst = Gem::DependencyInstaller.new
+    inst = Gem::DependencyInstaller.new(**options)
     inst.install name, version
     inst.installed_gems
   end

tool/rbinstall.rb

@@ -721,7 +721,7 @@ install?(:ext, :comm, :gem) do
   directories = Gem.ensure_gem_subdirectories(gem_dir, :mode => $dir_mode)
   prepare "bundle gems", gem_dir, directories
   Dir.glob(srcdir+'/gems/*.gem').each do |gem|
-    Gem.install gem
+    Gem.install gem, install_dir: with_destdir(Gem.dir)
     gemname = Pathname(gem).basename
     puts "#{" "*30}#{gemname}"
   end