Teaching cmake to always link in tcmalloc if it finds it.

Also renaming --enable-perftools to --enable-perftool-debug to
indicate that the switch is only relevant for debugging the heap. It's
not needed to pick up tcmalloc for better performance.

--with-perftools can still (and always) be used to give a hint where
to find the libraries.

With the threading, using tcmalloc improves memory usage on FreeBSD
significantly when running on a trace. If it fixes the live problems,
remains to be seen ...

src/ID.cc

@@ -372,7 +372,7 @@ ID* ID::Unserialize(UnserialInfo* info)
 
 		Ref(id);
 		global_scope()->Insert(id->Name(), id);
-#ifdef USE_PERFTOOLS
+#ifdef USE_PERFTOOLS_DEBUG
 		heap_checker->IgnoreObject(id);
 #endif
 		}