Allow master to build on Fedora with the recent threading changes

Allow master to build on Fedora with the recent threading changes

The recent threading changes have broken the build on Fedora,
and presumably other Red Hat based distributions. This adds an explicit
"-lpthread" to the linker command line and allows the latest master to build
on Fedora. I've also tested this on Ubuntu and it builds fine there.

Signed-off-by: Kyle Mestery <kmestery@cisco.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ed Maste <emaste@freebsd.org>