When linking the demo applications statically with pcap,
the CMake build system does not link with other libraries
that are already linked with libpcap
(-lnl-genl-3 -lnl-3 -ldbus-1 -pthread).

To fix this, the build system uses pcap-config:
pcap-config --libs --static

Resolves:
#187

Change-Id: I90d1c8997ca089d5c12dd6bfd276fe04cba572f7
Signed-off-by: Romain Naour <romain.naour@gmail.com>