From Lifeng Sun:
Since ruby-1.9.1 the header files are installed in a sub-directory, i.e.
"include/ruby-1.9.1/", and CONFIG["includedir"] is no longer the right
place to find the headers. 
I also fixed the the ruby library dectection on system with multiple
versions of ruby, but it's only a dirty hack for linux and several other
unix variants.


git-svn-id: https://root.cern.ch/svn/root/branches/v5-34-00-patches@48968 27541ba8-7e3a-0410-8455-c3a389f83636