CUDA: Fix default compiler flag initialization

Since commit v3.7.0-rc1~392^2 (Honor CMAKE_<LANG>_FLAGS[_<CONFIG>]_INIT
set in toolchain files, 2016-07-05) our convention is to initialize
compiler flag variables via `string(APPEND)` rather than `set()`.
Fix the convention for `CMAKE_CUDA_FLAGS[_<CONFIG>]_INIT`.

Modules/Compiler/NVIDIA-CUDA.cmake

@@ -10,11 +10,11 @@ set(CMAKE_SHARED_LIBRARY_CREATE_CUDA_FLAGS -shared)
 set(CMAKE_INCLUDE_SYSTEM_FLAG_CUDA -isystem=)
 set(CMAKE_CUDA_COMPILE_OPTIONS_VISIBILITY -Xcompiler=-fvisibility=)
 
-set(CMAKE_CUDA_FLAGS_INIT " ")
-set(CMAKE_CUDA_FLAGS_DEBUG_INIT " -g")
-set(CMAKE_CUDA_FLAGS_MINSIZEREL_INIT " -Os -DNDEBUG")
-set(CMAKE_CUDA_FLAGS_RELEASE_INIT " -O3 -DNDEBUG")
-set(CMAKE_CUDA_FLAGS_RELWITHDEBINFO_INIT " -O2 -g -DNDEBUG")
+string(APPEND CMAKE_CUDA_FLAGS_INIT " ")
+string(APPEND CMAKE_CUDA_FLAGS_DEBUG_INIT " -g")
+string(APPEND CMAKE_CUDA_FLAGS_MINSIZEREL_INIT " -Os -DNDEBUG")
+string(APPEND CMAKE_CUDA_FLAGS_RELEASE_INIT " -O3 -DNDEBUG")
+string(APPEND CMAKE_CUDA_FLAGS_RELWITHDEBINFO_INIT " -O2 -g -DNDEBUG")
 
 set(CMAKE_CUDA98_STANDARD_COMPILE_OPTION "")
 set(CMAKE_CUDA98_EXTENSION_COMPILE_OPTION "")