* 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/trunk@51812 b2dd03c8-39d4-4d8f-98ff-823fe69b080e