Recent spec file changes ensure that in distro situations, netcf
and libvirt will link against the same libnl in order to avoid
dumping core.  But for every-day development, if you use F17 and
have the libnl3-devel headers available, libvirt was blindly
linking against libnl3 even though F17 netcf still links against
libnl1, making testing a self-built binary on F17 impossible.

By making configure a little bit smarter, we can avoid this
situation - we merely skip the probe of libnl-3 if we can prove
that netcf is still using libnl-1.  I intentionally wrote the
test so that we still favor libnl-3 if netcf is not installed or
if we couldn't use ldd to determine things.

Defaults being what they are, someone will invariably complain
that our smarts were wrong.  Never fear - in that case, just run
./configure LIBNL_CFLAGS=..., where the fact that you set
LIBNL_CFLAGS (even to the empty string) will go back to probing
for libnl-3, regardless of netcf's choice.

* configure.ac (LIBNL): Don't probe libnl3 if netcf doesn't use it.