fixed compilation with Thrust versions >= 2

Closes #22

Thrust now has libcudacxx as dependency, regardless of device system choice. Its
location can now be specified with the LIBCUDACXX_ROOT variable. If a root for
Thrust is specified, and LIBCUDACXX_ROOT is left empty, SCons will look for it
in the Thrust repository. Clone Thrust with submodules to make that work.