Suffix array now compiles in cudpp using cmake on Linux

Hey Leyuan,

Take a look at this change please. I got SA compiling on cudpp now.  Can you try it and see if it compiles for you too?  Also, be sure you're using the mgpu and cub checkouts inside /ext and NOT ones in your local path.  You'll probably need to pull it.

git submodule init
git submodule update

or something.  We should sync with Mark to see if there's a better way of including these part in cmake.  It's kind of hacky now, and also since mgpu requires >= compute 2.0, CUDPP will no longer work on older cards with the way I've setup cmake.

 "Hopefully" it's just a matter of flipping the switch in compress_app.cu and everything will magically work as planned :D