Tweaking Fedora Source RPM builds

[Philip Prindeville <philipp_subx redfish-solutions com>]

I'm trying to build a slightly patched version of the x11-xorg source
RPM with the latest Unichrome drivers built in...  And I have a generic
RPM question.

If I don't want to build in /usr/src/redhat/ but instead have an NFS mounted
directory that I share amongst a variety of machines called /home/src/ 
instead,
how would I do a "rpmbuild -bc" but override the directory that gets used?

Do I need to redefine _topdir or $RPM_BUILD_ROOT or $RPM_BUILD_DIR
or what?

Thanks,

-Philip

P.S. Is there a good tutorial or O'Reilly book or whatever on becoming 
an RPM
   guru?