Download libpcap-X.Y.tar.gz from http://www.tcpdump.org/#latest-release
and verify the signature.

cd
# Make a copy of this directory.
cp -r ~/nmap/libpcap/NMAP_MODIFICATIONS .
tar xzvf libpcap-X.Y.tar.gz
cd nmap
rsync -rv --delete ~/libpcap-X.Y/ ~/nmap/libpcap/
# Remove some things we don't want to ship
rm -rf ~/nmap/libpcap/{rpcapd,testprogs,doc,tests}
# Apply patch to avoid those dirs
git apply ~/NMAP_MODIFICATIONS/0002-Disable-unnecessary-features.patch
git add -A libpcap
cd libpcap
autoconf
./configure
make
git add grammar.[ch] scanner.[ch]
cd ..
# Apply remaining patches.
git apply ~/NMAP_MODIFICATIONS/0001-Don-t-autogenerate-Lex-Yacc-files.patch
# Make changes as necessary and update the patch files
cd libpcap
autoconf
cd ..
git add -u libpcap
mv ~/NMAP_MODIFICATIONS libpcap/
git add libpcap/NMAP_MODIFICATIONS
git commit -m "Upgrade libpcap to X.Y."