build: require libnl-3 if netcf uses it

build: require libnl-3 if netcf uses it

Commits 9298bfb and f6c2951 both tried to make it possible to
select the correct libnl (1 vs. 3) according to what netcf
used, when both libraries are installed.  This works to avoid
libnl-3 when netcf used libnl-1.  But on the converse side, if
only libnl-1 development code is installed, while netcf uses
libnl-3, then configure happily uses libnl-1 anyways, leading
to a test failure:

$ VIR_TEST_DEBUG=1 ./virdrivermoduletest
TEST: virdrivermoduletest
 1) Test driver "network"                                             ... OK
 2) Test driver "storage"                                             ... OK
 3) Test driver "nodedev"                                             ... OK
 4) Test driver "secret"                                              ... OK
 5) Test driver "nwfilter"                                            ... OK
 6) Test driver "interface"
... lt-virdrivermoduletest: route/tc.c:973: rtnl_tc_register: Assertion
`0' failed.
Aborted

It's much nicer to prevent this at configure time, by requiring that
if we know what netcf used, then we want the same libnl version.  As
before, this can be bypassed by someone who knows what they are doing
by setting LIBNL_CFLAGS (perhaps useful to the rare person where the
build box has a different version of netcf than the installation box).

* configure.ac (LIBNL): If we can prove netcf used libnl-3, then
don't let configure succeed with libnl-1.

Signed-off-by: Eric Blake <eblake@redhat.com>