threads: Check w32threads dependencies at the configure stage

Also add warning comment about threading implementations without matching
atomics implementation to the atomics fallback implementation.

configure

@@ -1606,6 +1606,7 @@ atomics_gcc_if="sync_val_compare_and_swap"
 atomics_suncc_if="atomic_cas_ptr machine_rw_barrier"
 atomics_win32_if="MemoryBarrier"
 atomics_native_if_any="$ATOMICS_LIST"
+w32threads_deps="atomics_native"
 threads_if_any="$THREADS_LIST"
 
 # subsystems

libavutil/atomic.c

@@ -97,8 +97,11 @@ void *avpriv_atomic_ptr_cas(void * volatile *ptr, void *oldval, void *newval)
     return *ptr;
 }
 
-#else
+#else /* HAVE_THREADS */
 
+/* This should never trigger, unless a new threading implementation
+ * without correct atomics dependencies in configure or a corresponding
+ * atomics implementation is added. */
 #error "Threading is enabled, but there is no implementation of atomic operations available"
 
 #endif /* HAVE_PTHREADS */