add -lpthread on freebsd
add -lpthread on freebsd

closes gh-101

credit to @siggimoo for the patch
caidongyun/pyzmq,dash-dash/pyzmq,Mustard-Systems-Ltd/pyzmq,ArvinPan/pyzmq,swn1/pyzmq,yyt030/pyzmq,swn1/pyzmq,Mustard-Systems-Ltd/pyzmq,caidongyun/pyzmq,yyt030/pyzmq,ArvinPan/pyzmq,yyt030/pyzmq,swn1/pyzmq,Mustard-Systems-Ltd/pyzmq,caidongyun/pyzmq,dash-dash/pyzmq,ArvinPan/pyzmq,dash-dash/pyzmq