I think I found the solution here:
https://fedoraproject.org/wiki/PackagingDrafts/SELinux_Independent_Policy
BuildRequires: selinux-policy
%{?selinux_requires}
This way its expansion can be deferred at rebuild time, I suppose. But
don't we miss the other BuildRequires tags in this case?
We do:
make: /usr/share/selinux/devel/Makefile: No such file or directory
A different catch 22 now :(
Dridi