How to pass user defined parameters to rpmbuild to fill variables

rpmbuild -bb --define '_foobar Foo' somespecfile.spec

and refer it within the spec as %{_foobar}

(the _ is a convention, not a must)

could add this into the SPEC file:

%define _topdir %(echo $PWD)/

The closest you can get AFAIK are conditional builds, which allow you to do things like

rpmbuild -ba newpackage.spec --with feature

see http://rpm.org/user_doc/conditional_builds.html .