[Thrust] seems the installation style of cub has changed.

[cyk2018]

When I try your softmax examples, I get this error:

CMake Error at cmake/CubCudaConfig.cmake:4 (message):
  Building CUB as a standalone project is no longer supported.  Use the
  Thrust repo instead.
Call Stack (most recent call first):
  CMakeLists.txt:86 (include)

It looks like cub can install with your first part installation options.

And I find this path: thrust/build/headers/thrust，there are some files like this picture:

I try to update the makefile to replace the -IXXX with -I/thrust/build/headers/thrust, and the compilation success.

So I think we should add something like this in our documents.

Thanks for your contributions in open-source World!