[boost] patch for making shared_ptr compatible with root/cint

Hi, I would like to submit a patch for shared_ptr, to make it compatible with root/cint. cint is a C++ interpreter used by the root framework, which is popular in high energy physics. Details can be found at http://root.cern.ch/twiki/bin/view/ROOT/CINT. The patch uses one of the alternative "operator bool" definitions already in place when running with root/cint. Thanks in advance, Nils