[patch] Make std::tr1::shared_ptr thread-safe.

[Paolo Carlini]

Paolo Carlini wrote:

>Jonathan, please keep me updated, in case there is something I can do.
>  
>
Jonathan, for the configury bits (please accept in advance my excuses if
my suggestions are obvious), I think we should have something very
similar to the allocator bits ([GLIBCXX_ENABLE_ALLOCATOR], mt vs new) :
just go through the various available compare_and_swap and select the
compare_and_swap way as default for such targets, otherwise, for all the
other targets, the solution using empty critical sections as per
Alexander and Peter messages.

Paolo.