build: use pkg-config for libpcap

build: use pkg-config for libpcap

If building statically, with libpcap built with libnl support, linking
will fail, as the compiler won't be able to find the libnl symbols
since static libraries don't contain dependency information.  To fix
this, use pkg-config to find the flags for linking libpcap, since the
pkg-config files contain the neccesary dependency information.

autoconf will add code to the configure script for initializing
pkg-config the first time it seems PKG_CHECK_MODULES, so make the
libnfnetlink check the first one in the script, so the initialization
code is run unconditionally.

Signed-off-by: Alyssa Ross <hi@alyssa.is>
Signed-off-by: Phil Sutter <phil@nwl.cc>