libpcap: canusb support needs pthreads

canusb support in libpcap uses pthreads but the configure script doesn't
account for -lpthread usage so it doesn't get added into LIBS which in
turn doesn't add the appropiate entry in pcap-config, causing build
failures for programs that link to libpcap that don't use pthreads
themselves (for example tcpdump).
Also add the conditional dependency on BR2_PACKAGE_LIBUSB for canusb.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>