By default (on RHEL), source RPMs are installed to /usr/src/redhat. This is nice, except that I don’t want to be running as root when building software.
rpm -i --relocate /usr/src/redhat=/home/build/RPMBUILD setuptools-0.6c9-1.src.rpm
The previous command will install the specified source rpm to a local directory under the “build” user. That makes it easy to tweak the .spec file, and then build the desired RPM.