merge revision(s) 39284: [Backport #7860]

	* tool/mkconfig.rb: remove prefix from rubyarchdir.
	  r39267 expands variables, it changes expansion timing,
	  breaks RbConfig::CONFIG["includedir"] and building
	  extension libraries with installed ruby.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39581 b2dd03c8-39d4-4d8f-98ff-823fe69b080e