Visual C++ :: Exported DLL Symbol Validity

I'm building a cross-platform library which links to some other 3rd party libraries at run time (i.e. on Windows, the other libs will be available as DLLs whereas on Linux / OS-X etc they'd be shared objects, which are similar). For the sake of argument, one of those libraries is called "jack".

Obviously, our app can't guarantee which version of the other libs will be on the user's system (or even that they'll be installed at all). So our code is littered with statements like this:-

Code:
if (!jack_port_type_get_buffer_size) {
warning << _("This version of JACK is old - you should upgrade to a newer version") << endmsg;
} else {
some_var = jack_port_type_get_buffer_size();
}

We link to the latest version of jack, where that symbol is declared like so:-

Code:
size_t jack_port_type_get_buffer_size();

One problem is that it doesn't even seem to be an exported symbol (although that wouldn't affect the other platform builds). But apart from that, our customer might have an old copy of jack installed or even no copy! We seem to be making the assumption that if our customer has an up-to-date version, jack_port_type_get_buffer_size will be set to a valid address - but in all other case it'll be magically set to zero (there's nothing in our code that sets it to zero).