I noticed rpmbuild (-bb and --buildroot options) creates the .rpm in different locations depending of what OS are you using:
- GNU/Linux Ubuntu <= 9.04: /usr/src/rpm/...
 - GNU/Linux Ubuntu >= 9.10: /home/rpmbuild/...
 - GNU/Linux Fedora: /usr/src/redhat/...
 
So how can I set manually the destination folder for all OS?