Cross-installing gems

Obvisouly I want post-install.rb to write in ./ipkg, as setup.rb does.
What is post-install.rb trying to do?

I am trying to prepare a ruby-gems package for the iPAQ (armv4l
architecture, linux familiar distro)

$ uname -a
Linux chmeee #7 PREEMPT Sat Nov 19 23:16:29 CET 2005 i686
$ cd iPAQ/Ruby
$ ls
ruby-1.8.5/ # contains functional, cross-compiled Ruby for arm
$ cd rubygems-0.9.0/
$ ruby -I …/ruby-1.8.5 -rfake -rrbconfig setup.rb config
$ ruby -I …/ruby-1.8.5 -rfake -rrbconfig setup.rb install
As of RubyGems 0.8.0, library stubs are no longer needed.
Searching $LOAD_PATH for stubs to optionally delete (may take a
No library stubs found.

Successfully built RubyGem
Name: sources
Version: 0.0.1
File: sources-0.0.1.gem
hook /mnt/sda2/kero/iPAQ/Ruby/rubygems-0.9.0/./post-install.rb failed:
You don’t have write permissions into the /usr/lib/ruby/gems/1.8
Try ‘ruby setup.rb --help’ for detailed usage.