Expand $destdir to enable rdoc plugins for rubygems

`Gem::InstallerUninstallerUtils#regenerate_plugins_for` assumes that
`plugins_dir` is an absolute path as same as the target plugin files.

Author: nobu

tool/rbinstall.rb

@@ -150,6 +150,7 @@ def $mflags.defined?(var)
   end
 
   $destdir ||= $mflags.defined?("DESTDIR")
+  $destdir = File.expand_path($destdir) if $destdir
   if $extout ||= $mflags.defined?("EXTOUT")
     RbConfig.expand($extout)
   end
@@ -688,9 +689,6 @@ class UnpackedInstaller < Gem::Installer
     def write_cache_file
     end
 
-    def generate_plugins
-    end
-
     def shebang(bin_file_name)
       path = File.join(gem_dir, spec.bindir, bin_file_name)
       first_line = File.open(path, "rb") {|file| file.gets}