Remove macro redefinition in CUDA kernels
Remove macro redefinition in CUDA kernels

The macros being set in `CUDA.hpp` and `occa/defines.h` are now
protected with `#ifndef`'s.

This addresses some of the issues in
<https://github.com/libocca/occa/issues/19>.

jkozdon/occa,libocca/occa,lcw/occa,libocca/occa,jkozdon/occa,jkozdon/occa,jkozdon/occa,jkozdon/occa,lcw/occa,lcw/occa,lcw/occa,libocca/occa,libocca/occa,lcw/occa,jkozdon/occa,lcw/occa