improve CMakeLists.txt

- Put some projects in ExternalProjects *(optional, 119b659)*
- Fix warning for CMake 2.8.12-2:
  - addition of ```\"``` against whitespace warning (mind the whitespace near the end of line 205)
  - CUDA_NVCC_FLAGS instead of CMAKE_CXX_FLAGS against undefined symbol error
  - remove the whitespace inside to get rid of the ```'``` characters which cause errors