%setup macro must be present in %prep to set variables required by
find-debuginfo script. Symlink is to place sources in nice
/usr/src/debug/%{name}-%{version} subdir instead of plain /usr/src/debug/core
(which can be ambiguous).
Additionally all packages need to have _builddir pointing at top src dir (in
core-dom0 it was dom0 subdir). And to cheat make about current dir (to have
%{name}-%{version} included in path) chdir must be done by shell, not make - so
can't use make -C.
This libs are required by both dom0 and VM so it's better to have it
separately. Previously in VM it was separate package, but dom0 have them
embedded in qubes-core-dom0, but qubes-core-vm-libs package was used to build
qubes-gui-dom0. Now we do not build all packages for all distros (especially do
not build core-vm package for dom0 distro, so gui-dom0 build fails), so make it
explicit which package is needed by which system.
