Use BOOST_EXPLICIT_OPERATOR_BOOL for optional

I often have the problem that when I change a std::wstring to boost::optional<std::wstring> and the variable is used as a parameter with Boost.Format, the result silently changes from the string contents to "1".

This change prevents implicit conversion to bool if the compiler supports explicit conversion operators.

test/Jamfile.v2

@@ -29,5 +29,6 @@ import testing ;
     [ compile-fail optional_test_ref_fail4.cpp ]
     [ compile-fail optional_test_inplace_fail.cpp ]
     [ compile-fail optional_test_inplace_fail2.cpp ]
+    [ compile-fail optional_test_fail_implicit_bool_convert.cpp ]
   ;
 }