allow for additional root types

[Stefan Kost <ensonic hora-obscura de>]

hi,

yes! another mail ;)

The object hierarchy currently only has G_OBJECT and G_INTERFACE for the root 
types (see gtkdoc-scangobj:768).
This is usually sufficient, but bad, when a library has another root type.
E.g. GSteamer introduces GST_MINI_OBJECT - a lightweight G_OBJECT alike type.

It would be cool to allow something like 
--base-types="G_OBJECT,G_INTERFACE,GST_MINI_OBJECT" as part of SCANGOBJ_OPTIONS 
(in Makefile.am) to override the defaults.

If it sounds good, I post a patch.

Stefan