Merge pull request #5049 from rubygems/skip-destdir

Skip to set DESTDIR option when it's not provided for mswin platform

(cherry picked from commit 2bd3f00)

lib/rubygems/ext/builder.rb

@@ -30,7 +30,8 @@ def self.make(dest_path, results, make_dir = Dir.pwd)
     end
     make_program = Shellwords.split(make_program)
 
-    destdir = 'DESTDIR=%s' % ENV['DESTDIR']
+    # The installation of the bundled gems is failed when DESTDIR is empty in mswin platform.
+    destdir = (ENV['DESTDIR'] && ENV['DESTDIR'] != "") ? 'DESTDIR=%s' % ENV['DESTDIR'] : ''
 
     ['clean', '', 'install'].each do |target|
       # Pass DESTDIR via command line to override what's in MAKEFLAGS