I recently tried to build qt4-qtruby 2.1.0 against ruby 1.9 on Ubuntu Lucid.  I tried against Ubuntu's ruby 1.9.1 and
also against my own build of 1.9.2.  The cmake scripts kept reporting that they had failed to find ruby.

On investigation I noticed that there are two files with names that differ only in capitalisation:

cmake/modules/FindRUBY.cmake
cmake/modules/FindRuby.cmake

cmake was using FindRUBY but I wondered if it should have been using FindRuby.  Sure enough, when I replaced FindRUBY
with the contents of FindRuby, my build started working.

Perhaps builds on Windows work because Windows file systems are not case sensitive.  When the archive is extracted,
perhaps FindRUBY gets overwritten by FindRuby.  Once that has happened, any request by cmake for a file called findruby
(any capitalisation) will yield the contents of FindRuby and the build will work.