Only make CUDA -march workaround if compiling with CUDA target (#585)

Fixes #584.

cmake/CompilerOptions.cmake

@@ -135,12 +135,15 @@ function(set_arch_target optvar arch)
         endif()
     endif()
 
-    # Prefix architecture options with `-Xcompiler=` when compiling CUDA sources, i.e.
-    # with nvcc.
-    set(arch_opt_cuda_guarded)
-    foreach(opt ${arch_opt})
-        list(APPEND arch_opt_cuda_guarded "$<$<COMPILE_LANGUAGE:CUDA>:-Xcompiler=>${opt}")
-    endforeach()
-
-    set("${optvar}" "${arch_opt_cuda_guarded}" PARENT_SCOPE)
+    if (ARB_WITH_GPU)
+        # Prefix architecture options with `-Xcompiler=` when compiling CUDA sources, i.e.
+        # with nvcc.
+        set(arch_opt_cuda_guarded)
+        foreach(opt ${arch_opt})
+            list(APPEND arch_opt_cuda_guarded "$<$<COMPILE_LANGUAGE:CUDA>:-Xcompiler=>${opt}")
+        endforeach()
+
+        set("${optvar}" "${arch_opt_cuda_guarded}" PARENT_SCOPE)
+    endif()
+
 endfunction()