How to change --exec-prefix in %configure?

[Peter Lemenkov]

Hello All!
Subj.
I faced some difficulties. For some reason I need to add
--exec-prefix=/ to %configure. Firstly, I did it in he trivial way:

%configure --exec-prefix=/

but failed - default exec-prefix (which explicitly defines in that
macro) used instead of mine.

So far I've got simple question - how can I pass my custom exec-prefix
w/o copypasting whole %configure macro into my spec and fixing it a
little?
-- 
With best regards!