[C++-sig] Make threading support official

[troy d. straszheim]

troy d. straszheim wrote:
> 
> Take function new_class(...) in src/object.cpp:  this is called during 
> BOOST_PYTHON_MODULE(), and invoke.hpp doesn't know about it, therefore 
> nothing would be locked.
> 

Maybe this is a more practical example:


void set_foo_attr(object& obj, object& what)
{
    api::setattr(obj, "foo", what); // leave cpp in here, no unlock
}

BOOST_PYTHON_MODULE(m)
{
    def("set_foo_attr", &set_foo_attr);
}

 >>> class X: pass
 >>> x = X()
 >>> set_foo_attr(x, ['something'])
 >>> print x.foo
['something']


Here, enter/exitCPP would be called by invoke.hpp when set_foo_attr 
fires,   but not when api::setattr calls PyObject_SetAttrString.

-t