Fixed 7 MSVC2008 Compiler Warning (level 3) C4800: 'const int' : forcing value to bool 'true' or 'false' (performance warning)

- added explicit test "(xxx != 0)" to get a "bool" value out of the "int" expression
- see the MSDN recommandation http://msdn.microsoft.com/en-us/library/b6801kcy(v=vs.90).aspx

src/xpub.cpp

@@ -98,7 +98,7 @@ int zmq::xpub_t::xsetsockopt (int option_, const void *optval_,
         errno = EINVAL;
         return -1;
     }
-    verbose = *static_cast <const int*> (optval_);
+    verbose = (*static_cast <const int*> (optval_) != 0);
     return 0;
 }