the malloc debug system assumes single thread

lib/README.memoryleak

@@ -8,6 +8,13 @@ $Id$
              How To Track Down Suspected Memory Leaks in libcurl
              ===================================================
 
+Single-threaded
+
+  Please note that this memory leak system is not adjusted to work in more
+  than one thread. If you want/need to use it in a multi-threaded app. Please
+  adjust accordingly.
+
+
 Build
 
   Rebuild libcurl with -DMALLOCDEBUG (usually, rerunning configure with