How to make a selective spec file

[Eric Tanguy]

I would like to make only one spec taking into account the system build
is devel/fc5 or fc3/fc4.
How to make this ?
In devel/fc5 i need to have :
%configure CXXFLAGS="${RPM_OPT_FLAGS} -ffriend-injection"
and in fc3/fc4 i need to have : 
%configure

Thanks

Eric