now builds with VC9

git-svn-id: svn+ssh://svn.code.sf.net/p/quantlib/code/trunk@18505 8618b1d8-e22c-0410-b026-96a5dba3e089
Implicit conversions of shared_ptr to bool are now explicit.

The implicit conversion has been removed in C++11.
Thanks to Scott Condit.

git-svn-id: svn+ssh://svn.code.sf.net/p/quantlib/code/trunk@18506 8618b1d8-e22c-0410-b026-96a5dba3e089