MFC has an unobvious way to do this. Create a BOOL member variable that
holds the enable/disable desired state. When you want to change the
state you set the variable, but do not call any toolbar function.
Add a message handler for the UPDATE_COMMAND_UI message from the button.
In your message handler function call pCmdUI->Enable(m_bYourVariable);
MFC calls this function periodically during idle time.