The makefile assumes that the RPM build environment is $HOME/rpmbuild, which
is the default for RHEL 6 but not yet in RHEL 5.
Therefore on RHEL 5 one should add a line in its $HOME/.rpmmacros file to
set it, e.g. if your $HOME is /home/foo, you must define:
%_topdir /home/foo/rpmbuild