"have choice for" and Mandriva

I am trying to use wxGTK with my package.
With Mandriva (I am testing version 2008), I have a “have choice for” problem, result is:

have choice for wxGTK-devel: libwxgtk2.6-devel libwxgtk2.8-devel
have choice for wxGTK-devel: lib64wxgtk2.6-devel lib64wxgtk2.8-devel

Trusting the OBS documentation, I tried:

BuildRequires:	wxGTK-devel
%if 0%{?mandriva_version}
Prefer: 	libwxgtk2.8-devel 
Prefer: 	lib64wxgtk2.8-devel
%endif

Without success.
What is the key to success in providing my preference?