libpcap: fix static build

The libpcap package contained some interesting attempts to support a
static-only build, but it was not working:

 http://autobuild.buildroot.org/results/01038d3b970092d894d2bae80679247e65722785/build-end.log



In fact the configure.in of libpcap has provision to support
--enable-shared/--disable-shared, but the generated configure script
in the libpcap package has not been regenerated with the configure.in
changes.

So basically, enabling LIBPCAP_AUTORECONF=YES ensures that the
configure script gets generated, which brings us a working
--enable-shared / --disable-shared.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>