build: configure: Allow to compile tools without libnl

build: configure: Allow to compile tools without libnl

With libnl being made optional in commits 20a5e15443bf ("netsniff-ng:
Allow to compile without libnl") and c831bcda3e26 ("trafgen: Allow to
compile without libnl"), we can now compile netsniff-ng and trafgen
without libnl being present.

For now we don't consider libnl and libnl-route separately, meaning that
if libnl-route is not present, we disable libnl support entirely.

Also add a configure option to explicitely disable building netsniff-ng
and trafgen with libnl support.

Based on original patch by Vadim Kochan.

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>