[DFT][CMake] Check CUDA support with portFFT before using it as a target

* The portFFT backends targets Nvidia by default
* In some situations, this causes a failure whilst compiling (CUDA might not be installed)
* This checks the target is supported before use

cmake/FindCompiler.cmake

@@ -36,6 +36,9 @@ if(is_dpcpp)
   if(UNIX)
     set(UNIX_INTERFACE_COMPILE_OPTIONS -fsycl)
     set(UNIX_INTERFACE_LINK_OPTIONS -fsycl)
+    # Check if the Nvidia target is supported. PortFFT uses this for choosing default configuration.
+    check_cxx_compiler_flag("-fsycl -fsycl-targets=nvptx64-nvidia-cuda" dpcpp_supports_nvptx64)
+
     if(ENABLE_CURAND_BACKEND OR ENABLE_CUSOLVER_BACKEND)
       list(APPEND UNIX_INTERFACE_COMPILE_OPTIONS
         -fsycl-targets=nvptx64-nvidia-cuda -fsycl-unnamed-lambda)

src/dft/backends/portfft/CMakeLists.txt

@@ -36,7 +36,11 @@ if (IS_DPCPP AND UNIX AND NOT FOUND_TARGETS)
   set(TARGETS_LINK_OPTIONS -fsycl-unnamed-lambda)
 
   # spir64 must be last in the list due to a bug in dpcpp 2024.0.0
-  set(TARGETS_TRIPLES "nvptx64-nvidia-cuda,spir64")
+  if(dpcpp_supports_nvptx64)
+    set(TARGETS_TRIPLES "nvptx64-nvidia-cuda,spir64")
+  else()
+    set(TARGETS_TRIPLES "spir64")
+  endif()
 
   if (NOT (HIP_TARGETS STREQUAL ""))
     set(TARGETS_TRIPLES amdgcn-amd-amdhsa,${TARGETS_TRIPLES})
@@ -45,6 +49,8 @@ if (IS_DPCPP AND UNIX AND NOT FOUND_TARGETS)
   else()
     message(WARNING "Can't enable hip backend, HIP_TARGETS has not been set.")
   endif()
+
+  message(STATUS "portFFT target triple set to ${TARGETS_TRIPLES}")
 
   list(APPEND TARGETS_COMPILE_OPTIONS -fsycl-targets=${TARGETS_TRIPLES})
   list(APPEND TARGETS_LINK_OPTIONS -fsycl-targets=${TARGETS_TRIPLES})