merge revision(s) 39579: [Backport #7991]

	* lib/rubygems/ext/ext_conf_builder.rb (Gem::Ext::ExtConfBuilder.build):
	  fix for unusal cases again.  install to a temporary directory once
	  and move instaled files to the destination directory, if it is same
	  as the current directory.  [Bug #7698]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39791 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
merge revision(s) 39590,39592: [Backport #7991]

	* lib/rubygems/ext/ext_conf_builder.rb (Gem::Ext::ExtConfBuilder.build):
	  clear DESTDIR so RUBYARCHDIR and RUBYLIBDIR are not be overrdden.
	  [Bug #7698]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39792 b2dd03c8-39d4-4d8f-98ff-823fe69b080e