Problem building a packing list when build process uses ruby, rake, gems

[Mike Durian]

I'm having difficulty building a packing list for sipconfig.  It's
install process makes use of ruby and is dependant on rake and gems.
I can install normally if I don't define PREFIX, but when I do the
following:
make install PREFIX=/var/tmp/$(make -V PORTNAME)

I get an error from rake:
gmake[2]: Entering directory 
`/usr/ports/net/sipxconfig/work/sipxconfig-3.6.0/agent'
builddir=`pwd`; \
        cd . && /usr/local/bin/rake builddir="$builddir" package && cd
/usr/local/lib/ruby/site_ruby/1.8/rubygems.rb:204:in `report_activate_error': 
Could not find RubyGem rake (> 0) (Gem::LoadError)
        from /usr/local/lib/ruby/site_ruby/1.8/rubygems.rb:141:in `activate'
        from /usr/local/lib/ruby/site_ruby/1.8/rubygems.rb:37:in 
`require_gem_with_options'
        from /usr/local/lib/ruby/site_ruby/1.8/rubygems.rb:31:in `require_gem'
        from /usr/local/bin/rake:17


Does anyone have a work-around for this, or must I use find to produce a
list of everything under /usr/local, do the install without defining PREFIX
and then see what has been added?

Thanks
mike