MFH: r499639

net-mgmt/dhcdrop: Fix build on FreeBSD >= 12.0 and unbreak STATIC option

Add missing libibverbs dependency when linking statically. On
FreeBSD >= 12.0 libpcap needs libibverbs:

/usr/bin/ld: error: undefined symbol: ibv_get_device_list

referenced by pcap-rdmasniff.c:370 (/usr/src/contrib/libpcap/pcap-rdmasniff.c:370)

pcap-rdmasniff.o:(rdmasniff_create) in archive /usr/lib/libpcap.a

http://beefy6.nyi.freebsd.org/data/120amd64-default/499421/logs/errors/dhcdrop-0.5_2.log

While here fix the STATIC option. Currently the port always links
statically since --disable-static-build is broken and also enables
static linking.

Reported by: antoine, pkg-fallout

Approved by: ports-secteam blanket