Fix extconf error with sdkroot=nil

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2435 f4f36d40-9719-0410-9868-dde021a23eda
Add packaging with productbuild, not packagemaker

Xcode 5 does not support PackageMaker.app.
we need to build installer packages(.pkg) with "productbuild" command.

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2436 f4f36d40-9719-0410-9868-dde021a23eda
Add missing file "product.plist" at r2436

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2437 f4f36d40-9719-0410-9868-dde021a23eda