How to add custom rpm-defines to rpmbuild while building in Koji?

[Peter Lemenkov]

Hello All!
In order to build cross-toolchain I need to pass additional --define
"binutils_target <name of my target>" to rpmbuild command-line. See
this spec, for example:

http://cvs.fedoraproject.org/viewvc/rpms/spu-binutils/devel/spu-binutils.spec?revision=1.10&view=markup

How can I do it?

-- 
With best regards!