building.py: append messed up objs list, use extend to avoid that

The return type of SConscript is a subclass of UserList. We should use
extend to keep the depth of the list is always 1 when concatenate
SConscript return values. Thanks prife for making me realize that this
is really a problem.

Reported-and-Tested-by: prife
Merge pull request #37 from grissiom/fix-building-py

building.py: append messed up objs list, use extend to avoid that