build: fix detection of netcf linked with libnl1

build: fix detection of netcf linked with libnl1

Commit 9298bfbcb introduced code to detect if netcf is linked with
libnl1, and to prefer libnl1 over libnl3 when this is the case.
This behaviour can be disabled by setting LIBNL_CFLAGS to any value,
including the empty string.
However, configure.ac sets LIBNL_CFLAGS to "" before attempting
libnl detection, so the libnl1 detection code is always disabled.
This caused issues on my f17 system where netcf is linked with libnl1
but libvirt got built with libnl3.

This commit removes the setting of the LIBNL_* variables to "" as
this does not appear to be needed. After this change, libnl1 is
used when building libvirt on my f17 system.