FindRUBY.cmake from qt4-qtruby-1.4.10.tgz uses RUBY_ARCH_DIR to find ruby headers. 

RUBY_ARCH_DIR is 'archdir' and 'rubyicludedir' from CONFIG.

With Ruby 1.9.1 archdir is the library of .rb files and rubyincludedir is empty. The include files are in
CONFIG['rubyhdrdir'].