[CMake] CXX and CUDACXX

[Robert Maynard]

The default implementation is to defer to  CUDA for selecting what
ever host compiler it would like. To make sure that CMake uses the
same CXX and CUDACXX compiler you will need to explicitly state that
either through the CUDAHOSTCXX env variable (
https://cmake.org/cmake/help/v3.12/envvar/CUDAHOSTCXX.html ) or with
CMAKE_CUDA_HOST_COMPILER on the initial configuration of a project.