build: link against -pthread

Please see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=773341 for
more information.  Libuv uses some of pthread's symbols, but it doesn't
link against it, which causes underlinking issues, especially in Ubuntu
where we use ld --as-needed.  The issue also shows up as warnings in
Debian's build logs.

PR-URL: https://github.com/libuv/libuv/pull/73
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>