add basic test for CUDA compiler detection

The tests follows the CXX analogue. The availability of a CUDA compiler
is checked in the same way as for Fortran.
revise Modules/CMakeDetermineCUDACompiler etc.

Common code for detecting the compiler has been moved to
CMakeDetermineCompiler and is provided by _cmake_find_compiler().

The environment variable CUDA_TOOLKIT_ROOT_DIR is no longer used as a
hint. Such hints are currently not supported by _cmake_find_compiler, a
possible way out would be to set _${lang}_COMPILER_HINTS from an
optional function argument.