This was reported by jstribny in ticket

I would really love to see an --define option as in rpmbuild for defining macros in the spec file for the to-be-generated SRPM.

This is especially usuful when creating Software Collections when you need to define the scl macro before SRPM is created. Now I use rpmbuild for this like that:

rpmbuild -bs --define 'scl myscl' myscl-package.spec

and I would like to create this SRPM using fedpkg:

fedpkg srpm --define 'scl myscl'

I think this enhancement should be added as it's possibly quite a common use-case and it doesn't feel right to install rpmbuild just for this.