merge revision(s) 39297: [Backport #8032]

	mkconfig.rb: clear exec_prefix

	* tool/mkconfig.rb: clear exec_prefix which may differ from prefix,
  before expanding rubyarchdir to remove prefix.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39616 b2dd03c8-39d4-4d8f-98ff-823fe69b080e