configure: netsniff-ng: libpcap is not a hard dependency

netsniff-ng uses libpcap only for high-level filter compiling.
Thus, let netsniff-ng not "hard-depend" on libpcap, but rather
disable filter compiling in case the user does not want to
install libpcap.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>