build - Fix make concurrency for libpcap

* Fix a make concurrency issue

libpcap: Adjustments after the import

- Add README.DELETED back.
- Adjust README.DRAGONFLY
- Add HAVE_NET_PF_PFVAR_H define to config.h which allows including
pfvar.h and if_pflog.h from the s

libpcap: Adjustments after the import

- Add README.DELETED back.
- Adjust README.DRAGONFLY
- Add HAVE_NET_PF_PFVAR_H define to config.h which allows including
pfvar.h and if_pflog.h from the system headers. This should be perhaps
upstreamed.
- Leave the PCAP_DONT_INCLUDE_PCAP_BPF_H define in so that we can make
use of it. Include the system's bpf.h header when defined.
- tzoff field has been removed from struct pcap

show more ...