It is because the netlink library has been moved to /usr/include/libnl3/netlink
Here is what I did in order to build it successfully (on ubuntu 14.04):
# cd RouteFlow && git checkout vandervecken
1. Append "libssl-dev" to DEPENDENCIES on build.sh:
2. Append "-lpthrea -lssl -lcrypto" to the line LIBS in configure.ac
so it looks like
LIBS="-lboost_thread -lboost_system -lboost_filesystem -lboost_program_options -lpthread -lssl -lcrypto"