Drop hardware_interference_size. Not implemented in any standard library

pocketfft_hdronly.h

@@ -611,11 +611,9 @@ template <typename T> struct aligned_allocator
 
 class thread_pool
   {
-#if __cpp_lib_hardware_interference_size >= 201603L
-    struct alignas(std::hardware_destructive_interference_size) worker
-#else
-    struct alignas(64) worker
-#endif
+    // A reasonable guess, probably close enough for most hardware
+    static constexpr size_t cache_line_size = 64;
+    struct alignas(cache_line_size) worker
       {
       std::thread thread;
       std::condition_variable work_ready;