how about this instead?

%if %{pkg_vcmp libnl3-devel >= 3.4}
%bcond_without dco
%else
%bcond_with    dco
%endif
[snip]
%if %{with dco}
  confopt=--enable-dco
%else
  confopt=--enable-iproute2
%endif

if you wanted to stick with pkgconfig

you could use something like

if pkg-config --atleast-version 3.4 libnl3-devel ; then
...
fi

then you save yourself the ugly parsing.

Please reject, this package is not done yet. It will become a multibuild package with the default build supporting iproute2 and an alternative build supporting dco.

I just accepted a submission and forwarded it to Factory. @bmwiedemann take a look.