Fix -lpthread build

[zloidemon]

cc -O2 -pipe -I/usr/local/include -L/usr/local/lib -fno-strict-aliasing -g -O2 -Wall -Wno-strict-aliasing -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -o blastbeat src/main.o src/utils.o src/config.o src/zmq.o src/http.o src/ssl.o src/spdy.o src/uwsgi.o src/websocket.o src/sessions_ht.o src/writequeue.o src/groups.o src/socketio.o src/cache.o src/pipe.o src/memory.o http-parser/http_parser.o -lzmq -lssl -lcrypto -lev -lz -luuid
/usr/local/lib/libzmq.so: undefined reference to `pthread_create'
gmake: *** [all] Error 1