[Vala] Use of package-custom.vala

["Nguyen Thai Ngoc Duy" <pclouds gmail com>]

Hi,

I see vapi/packages/*/*-custom.vala and assume it will be merged to
final vapi files. But I don't know how (or what tool) to do it. I'm
experimenting libgsf binding and need to make some custom binding in
addition to vapigen generated file.

Thanks.
-- 
Duy