widl: Added an option to set importlib search path.

[Jacek Caban]

Alexandre Julliard wrote:
> Thanks; my idea is that since we don't want to make widl load a tlb
> from a builtin dll, we'll need to ship the raw tlb, and it would make
> sense to ship it in the include dir since the lib dir already contains
> the builtin dll. And when building Wine itself, we'll need the raw tlb
> in order to build the includes, since we can't build the builtins
> before the includes, so in both cases we'll want to load the tlb from
> the include dir. Does that make sense?
>   
Yes, I was thinking about something like that. Perhaps we could store
them in something like include/tlb to separate it from standard includes.

Thanks,
    Jacek