Fix feature detection macro for hardware_destructive_interference_size

pocketfft_hdronly.h

@@ -611,7 +611,7 @@ template <typename T> struct aligned_allocator
 
 class thread_pool
   {
-#if __cplusplus >= 201703L
+#if __cpp_lib_hardware_interference_size >= 201603L
     struct alignas(std::hardware_destructive_interference_size) worker
 #else
     struct alignas(64) worker