debug mode performance patch

[Paolo Carlini]

... one more small issue:

>    Here is a new proposition with the deadlock issue fixed. I use auto_ptr to acquire it even if it is deprecated because unique_ptr is only available in gnu++0x mode and the library is not built in this mode, is it fine ?

It is suboptimal, in my opinion: auto_ptr has well known very serious design flaws and it would be better to stay away from it. Thus, if, as I hope, the pointer is used only in a *.cc file, the straightforward thing to do would be building the file in c++0x mode, as happens already for many other *.cc files. Hacking the Makefile.am in /src for this is very simple (then, again, 'autoreconf').

Paolo