WeakRef, compile fixes for MSW

[ATS]

Hello (Vadz), 

I worked with sorting out build problems with wxMSW/VC8, after including
wxWeakRef. It now seems to work fine, and removes the need for any additional
compiler switches (/vmm /vmg). 

A define wxUSE_WEAKREF was also added to setup.h

I posted a small patch for this on SF tracker around a week ago (followup to
tracker ID 1870445). It's a minor thing, and I'd feel better knowing this is 
resolved. 

Regards
// ATS