bug: setting ENABLE_CLANG_CUDA and BLT_CXX_STD=c++17 causes error

[johnbowen42]

Whenever a project sets ENABLE_CLANG_CUDA=On and BLT_CXX_STD=c++17, CMake errors out inside BLTSetupCUDA.cmake, because

CMake Error at blt_test/blt/cmake/thirdparty/BLTSetupCUDA.cmake:72 (message):
CMake's CUDA_STANDARD does not support C++17.`

This is caused by the variable CMAKE_CUDA_COMPILE_FEATURES not being set when clang cuda is being used. Proejcts should be able to use clang to compile cuda with C++17, if their clang cuda compiler supports this standard.