Use '-x' option instead of symlinks to compile *.cu file using gcc

Symlinks were in use in previous versions because I did not succeed in modifying cc_args in a relevant way.

My cc_args was:
  [.., '-x c++', ..]  # not working because the argument is understood as -x\ c++
instead of the correct version:
  [.., '-x', 'c++', ..]

Symlinks was a tempory solution.
setup.py displays '-- Compiling using *** --' before starting the compilation