The Windows README file is a bit different:

You just simply have to change the QTDIR environment variable to
the directory containing QT:

"4. How to make Ruby/Qt
After define QTDIR to the directory where Qt-lib(ver 2.xx) is
installed, in the directory where you extract ruby-qt2 package,
execute following commands:
ruby setup.rb config
ruby setup.rb setup
(Change to root if necessary)
ruby setup.rb install
To compile each module, in the directory containing extconf.rb,
execute...
make -f Makefile.ruby_qt2 swigall
and
make
makes module file such as qt2.so."