* tool/extlibs.rb (do_patch): let "patch" command change the
	  working directory and open the patch file there, instead of
	  spawn options, so that proper error message will be shown by the
	  command not just "chdir" or "open".


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@54431 b2dd03c8-39d4-4d8f-98ff-823fe69b080e