[Polly] Back out of GPU Codegen if NVPTX is not available

[philip.pfaffe]

Summary

When enabling GPU codegen in polly, CMake will fail if NVPTX is not a target
supported by the LLVM polly is being built against. In that case, GPU codegen
should be switched off.

[Meinersbur]

LGTM

CMakeLists.txt

 endif ()
 
 # Add path for custom modules
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${POLLY_SOURCE_DIR}/cmake")
 
 SET(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
 
 option(POLLY_ENABLE_GPGPU_CODEGEN "Enable GPGPU code generation feature" OFF)
+set(GPU_CODEGEN FALSE)
 if (POLLY_ENABLE_GPGPU_CODEGEN)
   # Do not require CUDA/OpenCL, as GPU code generation test cases can be run
   # without a CUDA/OpenCL library.
+  if ("NVPTX" IN_LIST LLVM_TARGETS_TO_BUILD)
     FIND_PACKAGE(CUDA)
     FIND_PACKAGE(OpenCL)
     set(GPU_CODEGEN TRUE)
-else(POLLY_ENABLE_GPGPU_CODEGEN)
-  set(GPU_CODEGEN FALSE)
+  else()
+    message(WARNING "The LLVM NVPTX target is required for GPU code generation")
+  endif()
 endif(POLLY_ENABLE_GPGPU_CODEGEN)
 
 
 # Support GPGPU code generation if the library is available.
 if (CUDA_FOUND)
   add_definitions(-DHAS_LIBCUDART)
   INCLUDE_DIRECTORIES( ${CUDA_INCLUDE_DIRS} )
 endif(CUDA_FOUND)