CUDA: Set CMAKE_CUDA_COMPILER_LOADED variable when language is enabled

We already do this for C, CXX, Fortran, etc.

Modules/CMakeCUDACompiler.cmake.in

@@ -11,6 +11,7 @@ set(CMAKE_CUDA_SIMULATE_VERSION "@CMAKE_CUDA_SIMULATE_VERSION@")
 set(CMAKE_CUDA_COMPILER_ENV_VAR "CUDACXX")
 set(CMAKE_CUDA_HOST_COMPILER_ENV_VAR "CUDAHOSTCXX")
 
+set(CMAKE_CUDA_COMPILER_LOADED 1)
 set(CMAKE_CUDA_COMPILER_ID_RUN 1)
 set(CMAKE_CUDA_SOURCE_FILE_EXTENSIONS cu)
 set(CMAKE_CUDA_LINKER_PREFERENCE 15)