mkconfig.rb: delete DESTDIR

* tool/mkconfig.rb (RbConfig): delete default DESTDIR set by
  --with-destdir.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49748 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/mkconfig.rb

@@ -81,6 +81,7 @@ File.foreach "config.status" do |line|
     when /^RUBY_SO_NAME$/; next vars[name] = (so_name = val).dup if $so_name
     when /^arch$/; if val.empty? then val = arch else arch = val end
     when /^sitearch$/; val = '$(arch)' if val.empty?
+    when /^DESTDIR$/; next
     end
     case val
     when /^\$\(ac_\w+\)$/; next