Thread.ResourceCountKey: Survive being used with old pike binaries.

Old pike 8.0 binaries do not have Pike.DestructImmediate, and it is
not meaningful to attempt to kludge around it, as the corresponding
program flag is not propagated with them anyway.

lib/modules/Thread.pmod

@@ -879,8 +879,9 @@ optional class Farm
 //!   @[ResourceCount], @[MutexKey]
 //!
 optional class ResourceCountKey {
-
-  private inherit __builtin.DestructImmediate;
+#if constant(Pike.DestructImmediate)
+  private inherit Pike.DestructImmediate;
+#endif
 
   /*semi*/private ResourceCount parent;